[leetcode]75. Sort Colors三色排序

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

Example:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

思路：

fb的高频题，边界处理很容易出bug，要小心

扫一遍，maintain四个区域，

0 0  |  1 1 | XXXX |2 2
[0, zero) = 0
[zero, i) = 1
[i, two] = unchecked elements 
(two, len-1] = 2 

代码：

/**
0 0  | 1 | xxx | 2

if x == 1:   
0 0  | 1 | 1 xx | 2
           ^
0 0  | 1 | 1 xx | 2     
             ^

if x == 2: 
0 0  | 1 | 2xx | 2
           ^i    ^two
0 0  | 1 | x x 2| 2
           ^i  ^two

if x == 0: 
0 0  | 1 | 0 xx | 2
  ^zero    ^i
0 0  | 0 | 1 xx | 2
           ^i
**/

class Solution {
    public void sortColors(int[] nums) {
        // corner
        if(nums == null || nums.length == 0) return ;
        
        int zero = -1; //  nums[0...zero] = 0
        int two = nums.length; // nums[two...nums.length-1] = 2
        int i = 0;
        
        while(i < two){
            if(nums[i] == 1){
                i++;
            }else if(nums[i] == 2){
                two--; // to reserve a room 
                swap(nums, i , two); 
            }else{  // nums[i] == 0
                zero++;  // to reserve a room
                swap(nums, i , zero); 
                i++;
            }
        }   
    }  
    private void swap(int[] nums, int a, int b){
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
}