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.

Example:

Given [1, 0, 1, 2], sort it in-place to [0, 1, 1, 2].

Solution:

We could use two pointers to solve this question. One pointer is pointing from the front and the other one is pointing from the back. We swap all 0s to the left of the front pointer and all 2s to the right of the back pointer, then all 1s would remain in the center.

Code:

class Solution {
    /**
     * @param nums: A list of integer which is 0, 1 or 2 
     * @return: nothing
     */
    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] == 0) {
                swap(nums, i, left);
                left++;
                i++;
            } else if (nums[i] == 2) {
                swap(nums, i, right);
                right--;
            } else {
                i++;
            }

        }
    }

    private void swap(int[] nums, int a, int b) {
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
}