75. Sort Colors

Question

LeetCode Problem

Given an array nums 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.
We will use the integers 0 , 1 , and 2 to represent the color red, white, and blue, respectively.
You must solve this problem without using the library's sort function.
Example 1:

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

Example 2:

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

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is either 0 , 1 , or 2 .
  Follow up: Could you come up with a one-pass algorithm using only constant extra space?

Solutions

define 4 areas:

• [0, left) : red
• [left, mid) : white
• [mid, right): unknown
• [mid, m): blue

void sortColor(vector<int>& nums) {
	int m = static_cast<int>(nums.size());
	int left = 0;
	int mid = 0;
	int right = m - 1;
	while (mid <= right) {
		if (nums[mid] == 0) {
			swap(nums[left], nums[mid]);
			left++;
			mid++;
		} else if (nums[mid] == 1) {
			mid++;
		} else if (nums[mod] == 2) {
			swap(nums[right], nums[mid]);
			right--;
		}
	}
}