LeetCode Topics

Array

0001-two-sum
0004-median-of-two-sorted-arrays
0011-container-with-most-water
0016-3sum-closest
0018-4sum
0026-remove-duplicates-from-sorted-array
0033-search-in-rotated-sorted-array
0034-find-first-and-last-position-of-element-in-sorted-array
0035-search-insert-position
0036-valid-sudoku
0039-combination-sum
0046-permutations
0048-rotate-image
0049-group-anagrams
0051-n-queens
0053-maximum-subarray
0054-spiral-matrix
0056-merge-intervals
0057-insert-interval
0063-unique-paths-ii
0064-minimum-path-sum
0068-text-justification
0073-set-matrix-zeroes
0074-search-a-2d-matrix
0078-subsets
0079-word-search
0088-merge-sorted-array
0090-subsets-ii
0105-construct-binary-tree-from-preorder-and-inorder-traversal
0106-construct-binary-tree-from-inorder-and-postorder-traversal
0108-convert-sorted-array-to-binary-search-tree
0118-pascals-triangle
0119-pascals-triangle-ii
0120-triangle
0123-best-time-to-buy-and-sell-stock-iii
0128-longest-consecutive-sequence
0130-surrounded-regions
0134-gas-station
0135-candy
0136-single-number
0137-single-number-ii
0139-word-break
0149-max-points-on-a-line
0150-evaluate-reverse-polish-notation
0153-find-minimum-in-rotated-sorted-array
0162-find-peak-element
0188-best-time-to-buy-and-sell-stock-iv
0198-house-robber
0200-number-of-islands
0209-minimum-size-subarray-sum
0212-word-search-ii
0215-kth-largest-element-in-an-array
0217-contains-duplicate
0221-maximal-square
0228-summary-ranges
0238-product-of-array-except-self
0268-missing-number
0274-h-index
0283-move-zeroes
0289-game-of-life
0300-longest-increasing-subsequence
0322-coin-change
0334-increasing-triplet-subsequence
0373-find-k-pairs-with-smallest-sums
0380-insert-delete-getrandom-o1
0399-evaluate-division
0427-construct-quad-tree
0452-minimum-number-of-arrows-to-burst-balloons
0502-ipo
0526-beautiful-arrangement
0605-can-place-flowers
0643-maximum-average-subarray-i
0658-find-k-closest-elements
0724-find-pivot-index
0741-cherry-pickup
0746-min-cost-climbing-stairs
0898-transpose-matrix
0909-snakes-and-ladders
0954-maximum-sum-circular-subarray
0961-n-repeated-element-in-size-2n-array
1004-max-consecutive-ones-iii
1018-binary-prefix-divisible-by-5
1127-last-stone-weight
1184-car-pooling
1200-minimum-absolute-difference
1207-unique-number-of-occurrences
1431-kids-with-the-greatest-number-of-candies
1458-sort-integers-by-the-number-of-1-bits
1493-longest-subarray-of-1s-after-deleting-one-element
1559-cherry-pickup-ii
1679-max-number-of-k-sum-pairs
1732-find-the-highest-altitude
1877-minimize-maximum-pair-sum-in-array
2110-number-of-smooth-descent-periods-of-a-stock
2144-maximum-difference-between-increasing-elements
2215-find-the-difference-of-two-arrays
2227-sum-of-subarray-ranges
2300-successful-pairs-of-spells-and-potions
2352-equal-row-and-column-pairs
2435-paths-in-matrix-whose-sum-is-divisible-by-k
3010-divide-an-array-into-subarrays-with-minimum-cost-i
3074-apple-redistribution-into-boxes
3314-construct-the-minimum-bitwise-array-i
3507-minimum-pair-removal-to-sort-array-i
3512-minimum-operations-to-make-array-sum-divisible-by-k
3606-coupon-code-validator
3610-find-x-sum-of-all-k-long-subarrays-i
3637-trionic-array-i
3651-minimum-cost-path-with-teleportations

Two Pointers

0005-longest-palindromic-substring
0011-container-with-most-water
0016-3sum-closest
0018-4sum
0019-remove-nth-node-from-end-of-list
0026-remove-duplicates-from-sorted-array
0061-rotate-list
0082-remove-duplicates-from-sorted-list-ii
0086-partition-list
0088-merge-sorted-array
0148-sort-list
0151-reverse-words-in-a-string
0160-intersection-of-two-linked-lists
0234-palindrome-linked-list
0283-move-zeroes
0295-find-median-from-data-stream
0344-reverse-string
0345-reverse-vowels-of-a-string
0392-is-subsequence
0443-string-compression
0647-palindromic-substrings
0658-find-k-closest-elements
1679-max-number-of-k-sum-pairs
1768-merge-strings-alternately
1877-minimize-maximum-pair-sum-in-array
2300-successful-pairs-of-spells-and-potions

Sorting

0016-3sum-closest
0018-4sum
0049-group-anagrams
0056-merge-intervals
0088-merge-sorted-array
0148-sort-list
0215-kth-largest-element-in-an-array
0217-contains-duplicate
0268-missing-number
0274-h-index
0295-find-median-from-data-stream
0389-find-the-difference
0452-minimum-number-of-arrows-to-burst-balloons
0502-ipo
0658-find-k-closest-elements
1184-car-pooling
1200-minimum-absolute-difference
1458-sort-integers-by-the-number-of-1-bits
1657-determine-if-two-strings-are-close
1679-max-number-of-k-sum-pairs
1877-minimize-maximum-pair-sum-in-array
2300-successful-pairs-of-spells-and-potions
3010-divide-an-array-into-subarrays-with-minimum-cost-i
3074-apple-redistribution-into-boxes
3606-coupon-code-validator

Hash Table

0001-two-sum
0003-longest-substring-without-repeating-characters
0012-integer-to-roman
0017-letter-combinations-of-a-phone-number
0030-substring-with-concatenation-of-all-words
0036-valid-sudoku
0049-group-anagrams
0073-set-matrix-zeroes
0076-minimum-window-substring
0105-construct-binary-tree-from-preorder-and-inorder-traversal
0106-construct-binary-tree-from-inorder-and-postorder-traversal
0127-word-ladder
0128-longest-consecutive-sequence
0133-clone-graph
0138-copy-list-with-random-pointer
0139-word-break
0146-lru-cache
0149-max-points-on-a-line
0160-intersection-of-two-linked-lists
0208-implement-trie-prefix-tree
0217-contains-duplicate
0268-missing-number
0380-insert-delete-getrandom-o1
0387-first-unique-character-in-a-string
0389-find-the-difference
0433-minimum-genetic-mutation
0893-all-nodes-distance-k-in-binary-tree
0961-n-repeated-element-in-size-2n-array
1023-time-based-key-value-store
1207-unique-number-of-occurrences
1657-determine-if-two-strings-are-close
1679-max-number-of-k-sum-pairs
2215-find-the-difference-of-two-arrays
2352-equal-row-and-column-pairs
3507-minimum-pair-removal-to-sort-array-i
3606-coupon-code-validator
3610-find-x-sum-of-all-k-long-subarrays-i

String

0003-longest-substring-without-repeating-characters
0005-longest-palindromic-substring
0006-zigzag-conversion
0008-string-to-integer-atoi
0012-integer-to-roman
0017-letter-combinations-of-a-phone-number
0022-generate-parentheses
0030-substring-with-concatenation-of-all-words
0049-group-anagrams
0067-add-binary
0068-text-justification
0071-simplify-path
0072-edit-distance
0076-minimum-window-substring
0079-word-search
0097-interleaving-string
0127-word-ladder
0139-word-break
0151-reverse-words-in-a-string
0168-excel-sheet-column-title
0171-excel-sheet-column-number
0208-implement-trie-prefix-tree
0211-design-add-and-search-words-data-structure
0212-word-search-ii
0224-basic-calculator
0257-binary-tree-paths
0344-reverse-string
0345-reverse-vowels-of-a-string
0387-first-unique-character-in-a-string
0389-find-the-difference
0392-is-subsequence
0399-evaluate-division
0412-fizz-buzz
0433-minimum-genetic-mutation
0443-string-compression
0647-palindromic-substrings
0812-rotate-string
1023-time-based-key-value-store
1071-greatest-common-divisor-of-strings
1143-longest-common-subsequence
1302-delete-characters-to-make-fancy-string
1456-maximum-number-of-vowels-in-a-substring-of-given-length
1657-determine-if-two-strings-are-close
1768-merge-strings-alternately
1994-minimum-number-of-swaps-to-make-the-binary-string-alternating
2580-circular-sentence
3174-minimum-number-of-changes-to-make-binary-string-beautiful
3451-string-compression-iii
3606-coupon-code-validator
3768-check-if-digits-are-equal-in-string-after-operations-i

Dynamic Programming

0005-longest-palindromic-substring
0022-generate-parentheses
0053-maximum-subarray
0062-unique-paths
0063-unique-paths-ii
0064-minimum-path-sum
0070-climbing-stairs
0072-edit-distance
0097-interleaving-string
0118-pascals-triangle
0119-pascals-triangle-ii
0120-triangle
0123-best-time-to-buy-and-sell-stock-iii
0124-binary-tree-maximum-path-sum
0139-word-break
0188-best-time-to-buy-and-sell-stock-iv
0198-house-robber
0221-maximal-square
0300-longest-increasing-subsequence
0322-coin-change
0337-house-robber-iii
0338-counting-bits
0392-is-subsequence
0526-beautiful-arrangement
0647-palindromic-substrings
0741-cherry-pickup
0746-min-cost-climbing-stairs
0954-maximum-sum-circular-subarray
1013-fibonacci-number
1137-n-th-tribonacci-number
1143-longest-common-subsequence
1493-longest-subarray-of-1s-after-deleting-one-element
1559-cherry-pickup-ii
2110-number-of-smooth-descent-periods-of-a-stock
2435-paths-in-matrix-whose-sum-is-divisible-by-k
3651-minimum-cost-path-with-teleportations

Math

0002-add-two-numbers
0007-reverse-integer
0009-palindrome-number
0012-integer-to-roman
0048-rotate-image
0062-unique-paths
0067-add-binary
0069-sqrtx
0070-climbing-stairs
0149-max-points-on-a-line
0150-evaluate-reverse-polish-notation
0168-excel-sheet-column-title
0171-excel-sheet-column-number
0224-basic-calculator
0231-power-of-two
0268-missing-number
0380-insert-delete-getrandom-o1
0412-fizz-buzz
1013-fibonacci-number
1071-greatest-common-divisor-of-strings
1137-n-th-tribonacci-number
1523-count-odd-numbers-in-an-interval-range
1529-max-difference-you-can-get-from-changing-an-integer
1817-calculate-money-in-leetcode-bank
1925-count-square-sum-triples
2110-number-of-smooth-descent-periods-of-a-stock
2704-maximum-difference-by-remapping-a-digit
3512-minimum-operations-to-make-array-sum-divisible-by-k
3676-smallest-number-with-all-set-bits
3768-check-if-digits-are-equal-in-string-after-operations-i

Linked List

0002-add-two-numbers
0019-remove-nth-node-from-end-of-list
0021-merge-two-sorted-lists
0023-merge-k-sorted-lists
0024-swap-nodes-in-pairs
0025-reverse-nodes-in-k-group
0061-rotate-list
0082-remove-duplicates-from-sorted-list-ii
0086-partition-list
0092-reverse-linked-list-ii
0114-flatten-binary-tree-to-linked-list
0116-populating-next-right-pointers-in-each-node
0117-populating-next-right-pointers-in-each-node-ii
0138-copy-list-with-random-pointer
0146-lru-cache
0148-sort-list
0160-intersection-of-two-linked-lists
0203-remove-linked-list-elements
0206-reverse-linked-list
0234-palindrome-linked-list
0328-odd-even-linked-list
3507-minimum-pair-removal-to-sort-array-i

Recursion

0002-add-two-numbers
0021-merge-two-sorted-lists
0024-swap-nodes-in-pairs
0025-reverse-nodes-in-k-group
0203-remove-linked-list-elements
0206-reverse-linked-list
0224-basic-calculator
0231-power-of-two
0234-palindrome-linked-list
1013-fibonacci-number

Sliding Window

0003-longest-substring-without-repeating-characters
0030-substring-with-concatenation-of-all-words
0076-minimum-window-substring
0209-minimum-size-subarray-sum
0643-maximum-average-subarray-i
0658-find-k-closest-elements
1004-max-consecutive-ones-iii
1456-maximum-number-of-vowels-in-a-substring-of-given-length
1493-longest-subarray-of-1s-after-deleting-one-element
3610-find-x-sum-of-all-k-long-subarrays-i

Binary Search

0004-median-of-two-sorted-arrays
0033-search-in-rotated-sorted-array
0034-find-first-and-last-position-of-element-in-sorted-array
0035-search-insert-position
0069-sqrtx
0074-search-a-2d-matrix
0153-find-minimum-in-rotated-sorted-array
0162-find-peak-element
0209-minimum-size-subarray-sum
0268-missing-number
0270-closest-binary-search-tree-value
0300-longest-increasing-subsequence
0374-guess-number-higher-or-lower
0658-find-k-closest-elements
1004-max-consecutive-ones-iii
1023-time-based-key-value-store
2300-successful-pairs-of-spells-and-potions

Bit Manipulation

0067-add-binary
0078-subsets
0090-subsets-ii
0136-single-number
0137-single-number-ii
0190-reverse-bits
0191-number-of-1-bits
0201-bitwise-and-of-numbers-range
0231-power-of-two
0268-missing-number
0338-counting-bits
0389-find-the-difference
0526-beautiful-arrangement
1018-binary-prefix-divisible-by-5
1458-sort-integers-by-the-number-of-1-bits
3314-construct-the-minimum-bitwise-array-i
3676-smallest-number-with-all-set-bits

Memoization

0070-climbing-stairs
0139-word-break
1013-fibonacci-number
1137-n-th-tribonacci-number

Divide and Conquer

0004-median-of-two-sorted-arrays
0023-merge-k-sorted-lists
0053-maximum-subarray
0105-construct-binary-tree-from-preorder-and-inorder-traversal
0106-construct-binary-tree-from-inorder-and-postorder-traversal
0108-convert-sorted-array-to-binary-search-tree
0148-sort-list
0190-reverse-bits
0191-number-of-1-bits
0215-kth-largest-element-in-an-array
0427-construct-quad-tree
0954-maximum-sum-circular-subarray
1382-balance-a-binary-search-tree

Counting Sort

0274-h-index
0387-first-unique-character-in-a-string
1458-sort-integers-by-the-number-of-1-bits
1657-determine-if-two-strings-are-close

Design

0146-lru-cache
0155-min-stack
0173-binary-search-tree-iterator
0208-implement-trie-prefix-tree
0211-design-add-and-search-words-data-structure
0225-implement-stack-using-queues
0232-implement-queue-using-stacks
0295-find-median-from-data-stream
0380-insert-delete-getrandom-o1
0933-number-of-recent-calls
1023-time-based-key-value-store

Randomized

0380-insert-delete-getrandom-o1

Prefix Sum

0209-minimum-size-subarray-sum
0238-product-of-array-except-self
0724-find-pivot-index
1004-max-consecutive-ones-iii
1184-car-pooling
1732-find-the-highest-altitude

Greedy

0011-container-with-most-water
0134-gas-station
0135-candy
0334-increasing-triplet-subsequence
0452-minimum-number-of-arrows-to-burst-balloons
0502-ipo
0605-can-place-flowers
1382-balance-a-binary-search-tree
1529-max-difference-you-can-get-from-changing-an-integer
1877-minimize-maximum-pair-sum-in-array
1994-minimum-number-of-swaps-to-make-the-binary-string-alternating
2704-maximum-difference-by-remapping-a-digit
3074-apple-redistribution-into-boxes

Matrix

0036-valid-sudoku
0048-rotate-image
0054-spiral-matrix
0063-unique-paths-ii
0064-minimum-path-sum
0073-set-matrix-zeroes
0074-search-a-2d-matrix
0079-word-search
0130-surrounded-regions
0200-number-of-islands
0212-word-search-ii
0221-maximal-square
0289-game-of-life
0427-construct-quad-tree
0741-cherry-pickup
0898-transpose-matrix
0909-snakes-and-ladders
1559-cherry-pickup-ii
2352-equal-row-and-column-pairs
2435-paths-in-matrix-whose-sum-is-divisible-by-k
3651-minimum-cost-path-with-teleportations

Simulation

0054-spiral-matrix
0067-add-binary
0068-text-justification
0289-game-of-life
0412-fizz-buzz
0898-transpose-matrix
1184-car-pooling
2352-equal-row-and-column-pairs
3507-minimum-pair-removal-to-sort-array-i
3768-check-if-digits-are-equal-in-string-after-operations-i

String Matching

0572-subtree-of-another-tree
0812-rotate-string

Union Find

0128-longest-consecutive-sequence
0130-surrounded-regions
0200-number-of-islands
0399-evaluate-division

Stack

0071-simplify-path
0114-flatten-binary-tree-to-linked-list
0144-binary-tree-preorder-traversal
0145-binary-tree-postorder-traversal
0150-evaluate-reverse-polish-notation
0155-min-stack
0173-binary-search-tree-iterator
0224-basic-calculator
0225-implement-stack-using-queues
0232-implement-queue-using-stacks
0234-palindrome-linked-list
2227-sum-of-subarray-ranges

Tree

0098-validate-binary-search-tree
0101-symmetric-tree
0102-binary-tree-level-order-traversal
0103-binary-tree-zigzag-level-order-traversal
0104-maximum-depth-of-binary-tree
0105-construct-binary-tree-from-preorder-and-inorder-traversal
0106-construct-binary-tree-from-inorder-and-postorder-traversal
0108-convert-sorted-array-to-binary-search-tree
0110-balanced-binary-tree
0111-minimum-depth-of-binary-tree
0114-flatten-binary-tree-to-linked-list
0116-populating-next-right-pointers-in-each-node
0117-populating-next-right-pointers-in-each-node-ii
0124-binary-tree-maximum-path-sum
0144-binary-tree-preorder-traversal
0145-binary-tree-postorder-traversal
0173-binary-search-tree-iterator
0199-binary-tree-right-side-view
0226-invert-binary-tree
0236-lowest-common-ancestor-of-a-binary-tree
0257-binary-tree-paths
0270-closest-binary-search-tree-value
0337-house-robber-iii
0427-construct-quad-tree
0450-delete-node-in-a-bst
0530-minimum-absolute-difference-in-bst
0543-diameter-of-binary-tree
0572-subtree-of-another-tree
0637-average-of-levels-in-binary-tree
0700-search-in-a-binary-search-tree
0893-all-nodes-distance-k-in-binary-tree
0938-range-sum-of-bst
1161-maximum-level-sum-of-a-binary-tree
1382-balance-a-binary-search-tree

Depth-First Search

0079-word-search
0098-validate-binary-search-tree
0101-symmetric-tree
0104-maximum-depth-of-binary-tree
0110-balanced-binary-tree
0111-minimum-depth-of-binary-tree
0114-flatten-binary-tree-to-linked-list
0116-populating-next-right-pointers-in-each-node
0117-populating-next-right-pointers-in-each-node-ii
0124-binary-tree-maximum-path-sum
0130-surrounded-regions
0133-clone-graph
0144-binary-tree-preorder-traversal
0145-binary-tree-postorder-traversal
0199-binary-tree-right-side-view
0200-number-of-islands
0207-course-schedule
0210-course-schedule-ii
0211-design-add-and-search-words-data-structure
0226-invert-binary-tree
0236-lowest-common-ancestor-of-a-binary-tree
0257-binary-tree-paths
0270-closest-binary-search-tree-value
0337-house-robber-iii
0399-evaluate-division
0530-minimum-absolute-difference-in-bst
0543-diameter-of-binary-tree
0572-subtree-of-another-tree
0637-average-of-levels-in-binary-tree
0893-all-nodes-distance-k-in-binary-tree
0938-range-sum-of-bst
1161-maximum-level-sum-of-a-binary-tree
1382-balance-a-binary-search-tree

Breadth-First Search

0101-symmetric-tree
0102-binary-tree-level-order-traversal
0103-binary-tree-zigzag-level-order-traversal
0104-maximum-depth-of-binary-tree
0111-minimum-depth-of-binary-tree
0116-populating-next-right-pointers-in-each-node
0117-populating-next-right-pointers-in-each-node-ii
0127-word-ladder
0130-surrounded-regions
0133-clone-graph
0199-binary-tree-right-side-view
0200-number-of-islands
0207-course-schedule
0210-course-schedule-ii
0226-invert-binary-tree
0322-coin-change
0399-evaluate-division
0433-minimum-genetic-mutation
0530-minimum-absolute-difference-in-bst
0637-average-of-levels-in-binary-tree
0893-all-nodes-distance-k-in-binary-tree
0909-snakes-and-ladders
1161-maximum-level-sum-of-a-binary-tree

Binary Tree

0098-validate-binary-search-tree
0101-symmetric-tree
0102-binary-tree-level-order-traversal
0103-binary-tree-zigzag-level-order-traversal
0104-maximum-depth-of-binary-tree
0105-construct-binary-tree-from-preorder-and-inorder-traversal
0106-construct-binary-tree-from-inorder-and-postorder-traversal
0108-convert-sorted-array-to-binary-search-tree
0110-balanced-binary-tree
0111-minimum-depth-of-binary-tree
0114-flatten-binary-tree-to-linked-list
0116-populating-next-right-pointers-in-each-node
0117-populating-next-right-pointers-in-each-node-ii
0124-binary-tree-maximum-path-sum
0144-binary-tree-preorder-traversal
0145-binary-tree-postorder-traversal
0173-binary-search-tree-iterator
0199-binary-tree-right-side-view
0226-invert-binary-tree
0236-lowest-common-ancestor-of-a-binary-tree
0257-binary-tree-paths
0270-closest-binary-search-tree-value
0337-house-robber-iii
0450-delete-node-in-a-bst
0530-minimum-absolute-difference-in-bst
0543-diameter-of-binary-tree
0572-subtree-of-another-tree
0637-average-of-levels-in-binary-tree
0700-search-in-a-binary-search-tree
0893-all-nodes-distance-k-in-binary-tree
0938-range-sum-of-bst
1161-maximum-level-sum-of-a-binary-tree
1382-balance-a-binary-search-tree

Binary Search Tree

0098-validate-binary-search-tree
0108-convert-sorted-array-to-binary-search-tree
0173-binary-search-tree-iterator
0270-closest-binary-search-tree-value
0450-delete-node-in-a-bst
0530-minimum-absolute-difference-in-bst
0700-search-in-a-binary-search-tree
0938-range-sum-of-bst
1382-balance-a-binary-search-tree

Backtracking

0017-letter-combinations-of-a-phone-number
0022-generate-parentheses
0039-combination-sum
0046-permutations
0051-n-queens
0052-n-queens-ii
0077-combinations
0078-subsets
0079-word-search
0090-subsets-ii
0212-word-search-ii
0257-binary-tree-paths
0526-beautiful-arrangement

Queue

0225-implement-stack-using-queues
0232-implement-queue-using-stacks
0387-first-unique-character-in-a-string
0933-number-of-recent-calls
0954-maximum-sum-circular-subarray

Monotonic Queue

0954-maximum-sum-circular-subarray

Doubly-Linked List

0146-lru-cache
3507-minimum-pair-removal-to-sort-array-i

Merge Sort

0023-merge-k-sorted-lists
0148-sort-list

Heap (Priority Queue)

0023-merge-k-sorted-lists
0215-kth-largest-element-in-an-array
0295-find-median-from-data-stream
0373-find-k-pairs-with-smallest-sums
0502-ipo
0658-find-k-closest-elements
1127-last-stone-weight
1184-car-pooling
3507-minimum-pair-removal-to-sort-array-i
3610-find-x-sum-of-all-k-long-subarrays-i

Monotonic Stack

2227-sum-of-subarray-ranges

Quickselect

0215-kth-largest-element-in-an-array

Graph

0133-clone-graph
0207-course-schedule
0210-course-schedule-ii
0399-evaluate-division

Topological Sort

0207-course-schedule
0210-course-schedule-ii

Bitmask

0526-beautiful-arrangement

Trie

0139-word-break
0208-implement-trie-prefix-tree
0211-design-add-and-search-words-data-structure
0212-word-search-ii

Geometry

0149-max-points-on-a-line

Iterator

0173-binary-search-tree-iterator

Shortest Path

0399-evaluate-division

Combinatorics

0062-unique-paths
3768-check-if-digits-are-equal-in-string-after-operations-i

Number Theory

3768-check-if-digits-are-equal-in-string-after-operations-i

Hash Function

0572-subtree-of-another-tree

Enumeration

1925-count-square-sum-triples
3010-divide-an-array-into-subarrays-with-minimum-cost-i

Data Stream

0295-find-median-from-data-stream
0933-number-of-recent-calls

Ordered Set

3507-minimum-pair-removal-to-sort-array-i

Interactive

0374-guess-number-higher-or-lower