LeetCode: 75. Sort Colors

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75. Sort Colors

Given an array with n objects colored red, white or blue, sort them *https://en.wikipedia.org/wiki/In-place_algorithm[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?

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/**
 * Runtime: 0 ms, faster than 100.00% of Java online submissions for Sort Colors.
 *
 * Memory Usage: 35.2 MB, less than 99.21% of Java online submissions for Sort Colors.
 */
public void sortColors(int[] nums) {
    if (Objects.isNull(nums) || nums.length <= 1) {
        return;
    }
    int current = 0;
    int head = 0;
    int tail = nums.length - 1;
    while (current <= tail) {
        if (nums[current] == 0) {
            swap(nums, current, head);
            head++;
            current++;
        } else if (nums[current] == 2) {
            swap(nums, current, tail);
            tail--;
            // 这了不需要 current++，就是把这个数字重新排队一遍！
        } else {
            current++;
        }
    }
}

private void swap(int[] nums, int i, int j) {
    int temp = nums[i];
    nums[i] = nums[j];
    nums[j] = temp;
}