Color Sort

问题描述

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. 

解决思路

1. 计数排序；

2. 双指针法，一遍扫描；

程序

1. 普通版本

public class Solution {
    public void sortColors(int[] nums) {
        if (nums == null || nums.length == 0) {
            return ;
        }
        int begin = 0, end = nums.length - 1;
        while (true) {
            while (begin < end && nums[begin] == 0) {
                ++begin;
            }
            while (end > begin && nums[end] == 2) {
                --end;
            }
            if (begin >= end) {
                break ;
            }
            int p = end;
            while (p >= begin && nums[p] == 1) {
                --p;
            }
            if (p < begin) {
                break;
            }
            if (nums[p] == 0) {
                swap(nums, p, begin);
            } else {
                swap(nums, p, end);
            }
        }
    }
    
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

2. 精炼版本

public class Solution {
    public void sortColors(int[] nums) {
        if (nums == null || nums.length == 0) {
            return ;
        }
        int begin = 0, end = nums.length;
        for (int i = 0; i < end; i++) {
            if (nums[i] == 0) {
                swap(nums, i, begin);
                ++begin;
            } else if (nums[i] == 2) {
                --end;
                swap(nums, i, end);
                --i; // 交换前面的元素不确定为0或者1
            }
        }
    }
    
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}