75. Sort Colors

75. Sort Colors (Medium)

Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note: You are not suppose to use the library's sort function for this problem.

Example:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Follow up:

• A rather straight forward solution is a two-pass algorithm using counting sort.
  First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.
• Could you come up with a one-pass algorithm using only constant space?

Related Topics:
Array, Two Pointers, Sort

Similar Questions:

• Sort List (Medium)
• Wiggle Sort (Medium)
• Wiggle Sort II (Medium)

Solution 1.

// OJ: https://leetcode.com/problems/sort-colors/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    void sortColors(vector<int>& nums) {
        vector<int> cnt(3, 0);
        for (int n : nums) cnt[n]++;
        int i = 0;
        for (int j = 0; j < 3; ++j) {
            while (cnt[j]--) nums[i++] = j;
        }
    }
};

Solution 2.

// OJ: https://leetcode.com/problems/sort-colors/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    void sortColors(vector<int>& nums) {
        int r = 0, g = 0, b = 0;
        for (int n : nums) {
            if (n == 0) {
                nums[b++] = 2;
                nums[g++] = 1;
                nums[r++] = 0;
            } else if (n == 1) {
                nums[b++] = 2;
                nums[g++] = 1;
            } else nums[b++] = 2;
        }
    }
};