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75. Sort Colors

Medium

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]

Example 3:

Input: nums = [0]

Output: [0]

Example 4:

Input: nums = [1]

Output: [1]

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is 0, 1, or 2.

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

Solution

public class Solution {
    public void SortColors(int[] nums) {
        int zeroes = 0;
        int ones = 0;
        for (int i = 0; i < nums.Length; i++) {
            if (nums[i] == 0) {
                nums[zeroes++] = 0;
            } else if (nums[i] == 1) {
                ones++;
            }
        }
        for (int j = zeroes; j < zeroes + ones; j++) {
            nums[j] = 1;
        }
        for (int k = zeroes + ones; k < nums.Length; k++) {
            nums[k] = 2;
        }
    }
}

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