个人记录-LeetCode 75. Sort Colors

问题：
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.

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 an one-pass algorithm using only constant space?

先计数后排序的思路如同Follow up所述，比较简单，就不写代码了。
这里主要分析一下one-pass的方法，其思路说穿了就是：
将2号球全部丢到队列尾部，将0号球全部丢到队列前端。

代码示例：

public class Solution {
    public void sortColors(int[] nums) {
        //zeroIndex将记录下一个0号球放置的位置
        //secondIndex将记录下一个2号球放置的位置
        int zeroIndex = 0, secondIndex = nums.length-1;

        for (int i=0; i <= secondIndex; i++) {
            //保证i位置之前的2号全部移动到了末尾
            while (nums[i] == 2 && i < secondIndex) {
                swap(nums, i, secondIndex--);
            }
            //保证i位置之前的0号球全部移动到了前端
            while (nums[i] == 0 && i > zeroIndex) {
                swap(nums, i, zeroIndex++);
            }
        }
    }

    private void swap(int[] nums, int begin, int end) {
        int temp = nums[begin];
        nums[begin] = nums[end];
        nums[end] = temp;
    }
}