Sort Colors

Given an array withn_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 integers0,1, and2to represent the color red, white, and blue respectively.

Notice

You are not suppose to use the library's sort function for this problem. You should do it in-place (sort numbers in the original array).

Example

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

分析：

快速排序, partition出一个Index,然后左右各自递归排序

答案;

class Solution {
    /**
     * @param nums: A list of integer which is 0, 1 or 2 
     * @return: nothing
     */
    public void sortColors(int[] nums) {
        // write your code here
        if(nums == null || nums.length == 0)
            return;
        qsort(nums, 0, nums.length - 1);
    }

    public void qsort(int[] nums, int l, int r){
        if(l >= r) return;
        int index = partition(nums, l, r);
        qsort(nums, l, index-1);
        qsort(nums, index+1, r);
    }

    private int partition(int[] nums, int l, int r){
        int left = l, right = r;
        int pivot = nums[left];
        while(left < right){
            while(left < right && nums[right] >= pivot){
                right --;
            }
            nums[left] = nums[right];

            while(left < right && nums[left] <= pivot){
                left ++;
            }
            nums[right] = nums[left];
        }
        nums[left] = pivot;
        return left;
    }
}