📘 DSA Journey Catalog

Welcome to my DSA (Data Structures and Algorithms) journey! This repository is a personal log of the concepts I’ve learned, problems I’ve solved, and progress I’ve made while mastering DSA. It serves both as a revision notebook and a motivation tracker.

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🗓️ Progress Tracker

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📅 Day 1

• Topics Covered:
  • Binary Search
  • Pivot in Array

1. Find Peak in a Mountain Array

• ✅ Approach: Binary search for arr[mid] < arr[mid+1] to locate the increasing slope end.
• 💡 Method Used: peakIndexInMountainArray(vector<int>& arr)
• 💻 C++ Snippet:

while (low < high) {
    int mid = low + (high - low) / 2;
    if (arr[mid] < arr[mid + 1]) low = mid + 1;
    else high = mid;
}

2. Find First and Last Occurrence

• ✅ Approach: Two binary searches: one for the first and one for the last occurrence.
• 💡 Method Used: searchRange(vector<int>& nums, int target)
• 💻 C++ Snippet:

int binarySearch(bool findFirst) {
    int res = -1;
    while (low <= high) {
        int mid = (low + high) / 2;
        if (nums[mid] == target) {
            res = mid;
            findFirst ? high = mid - 1 : low = mid + 1;
        }
        else if (nums[mid] < target) low = mid + 1;
        else high = mid - 1;
    }
    return res;
}

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📅 Day 2

• Topics Covered:
  • Binary Search Applications

1. Search in Rotated Sorted Array

• ✅ Approach: Modified binary search using pivot logic.
• 💡 Method Used: search(vector<int>& nums, int target)

2. Square Root using Binary Search

• ✅ Approach: Binary search between 1 and x/2, compare mid*mid.
• 💡 Method Used: mySqrt(int x)

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📅 Day 3

• Topics Covered:
  • Strings
  • String Arrays

1. Valid Palindrome

• ✅ Approach: Two pointers from both ends, skip non-alphanumerics.

2. Reverse Words in a String

• ✅ Approach: Trim + reverse each word + reverse entire string.

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📅 Day 4

• Topics Covered:
  • Strings
  • Sliding Window

1. Implement strStr()

• ✅ Approach: Sliding window to compare substring matches.

2. Permutation in String

• ✅ Approach: Compare frequency maps of sliding window and target.

3. String Compression

• ✅ Approach: Use two pointers to modify array in-place.

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📅 Day 5

• Topics Covered:
  • 2D Arrays

1. Wave Print of a 2D Matrix

• ✅ Approach: Traverse columns alternatively top to bottom and bottom to top.

2. Spiral Print of a 2D Matrix

• ✅ Approach: Maintain 4 boundaries and iterate in spiral fashion.

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📅 Day 6

• Topics Covered:
  • 2D Arrays

1. Search a 2D Matrix

• ✅ Approach: Flatten the matrix logically and apply binary search.

2. Search a 2D Matrix II

• ✅ Approach: Start from top-right, eliminate row or column based on comparison.

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📅 Day 7

• Topics Covered:
  • Recursion

1. Sort an Array Using Recursion

• ✅ Approach: Pop elements recursively and insert them back in sorted order.
• 💻 C++ Snippet:

void insert(vector<int>& arr, int temp) {
    if (arr.empty() || arr.back() <= temp) {
        arr.push_back(temp);
        return;
    }
    int val = arr.back(); arr.pop_back();
    insert(arr, temp);
    arr.push_back(val);
}

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📅 Day 8

• Topics Covered:
  • Recursion

1. Binary Search (Recursive)

• ✅ Approach: Base case and recursive reduction of search space.

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📅 Day 9

• Topics Covered:
  • Recursion

1. Subsets

• ✅ Approach: Backtracking to include/exclude elements.

2. Subsets II

• ✅ Approach: Same as Subsets but skip duplicates using sorted array.

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📅 Day 10

• Topics Covered:
  • Recursion

1. Letter Combinations of a Phone Number

• ✅ Approach: Map each digit to possible characters and recurse to build combinations.
• 💻 C++ Snippet:

void backtrack(string& digits, int index, string& path, vector<string>& res, vector<string>& map) {
    if (index == digits.size()) {
        res.push_back(path);
        return;
    }
    for (char c : map[digits[index] - '0']) {
        path.push_back(c);
        backtrack(digits, index + 1, path, res, map);
        path.pop_back();
    }
}

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📅 Day 11

• Topics Covered:
  • Linked List

1. Reverse a Linked List

• ✅ Approach: Iteratively reverse the pointers using three variables.
• 💻 C++ Snippet:

ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;
    while (head) {
        ListNode* next = head->next;
        head->next = prev;
        prev = head;
        head = next;
    }
    return prev;
}

📅 Day 11

• Topics Covered:
  • Linked List

1. Reverse a Linked List II

• ✅ Approach: Reverse the part of linked list which from left to right and then connect the reversed linked list to the res of the elements. i.e left to prevright-> and right to x where x -> next = prevleft
• 💻 C++ Snippet:

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
        if (!head || left == right) return head;

        // Create a dummy node to simplify edge cases
        ListNode* dummy = new ListNode(0);
        dummy->next = head;

        // Step 1: Move to node before 'left'
        ListNode* prevLeft = dummy;
        for (int i = 1; i < left; ++i) {
            prevLeft = prevLeft->next;
        }

        // Step 2: Reverse from left to right
        ListNode* curr = prevLeft->next;      // Node at 'left'
        ListNode* prev = nullptr;
        ListNode* tail = curr;                // This will become the tail after reversing

        for (int i = 0; i < right - left + 1; ++i) {
            ListNode* nextTemp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = nextTemp;
        }

        // Step 3: Connect reversed part with remaining list
        prevLeft->next = prev;    // 'prev' is new head of reversed sublist
        tail->next = curr;        // 'curr' is the node after 'right'

        return dummy->next;
    }
};

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🎯 Goal

To build a strong foundation in core DSA concepts and solve a wide range of problems with increasing difficulty, preparing for technical interviews and competitive programming.

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🛠️ Tools Used

• Language: Python / C++
• Platform: LeetCode, Codeforces, GeeksforGeeks
• Version Control: Git + GitHub

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🚀 How to Use

Feel free to explore the daily logs, code snippets, and approaches. You can fork this repository and start your own DSA journey too!

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🙌 Let’s Connect!

If you're also on your DSA journey, let’s connect and grow together!