Sort Colors

Given an array with n objects colored red, white or blue, sort them 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.

/*

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.

Also, we can use three pointers with one pass solution. In each step during the iteration, we do either of three things. Swap with the left, swap with the right, or move forward.

time:O(n), space:O(1)

*/

public class Solution {

    public void sortColors(int[] nums) {

        if (nums == null || nums.length == 0) {

            return;

        }

        int left = 0;

        int right = nums.length - 1;

        int i = 0;

        while (i <= right) {

            if (nums[i] == 1) {

                i++;

            } else if (nums[i] == 0) {

                swap(nums, left, i); 

                left++;

                i++;

            } else {

                swap(nums, right, i);

                right--;

            }

        }

    }

    public void swap(int[] nums, int a, int b) {

        int tmp = nums[a];

        nums[a] = nums[b];

        nums[b] = tmp;

    }

}