sort colors 三色颜色排序 快速排序

Given an array nums 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.

Follow up:

• Could you solve this problem without using the library's sort function?
• Could you come up with a one-pass algorithm using only O(1) constant space?

 

Example 1:

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

Example 2:

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

Example 3:

Input: nums = [0]
Output: [0]

Example 4:

Input: nums = [1]
Output: [1]

 

思路：不是比较元素，是比较index。元素只要跟基准比较就行了。

中间的if不动了，此时还是left <= right。双方换一下

class Solution {
    public void sortColors(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        //和1比较
        while (left <= right) {
            while (left < right && nums[left] < 1) {
                left++;
            }
            
            while (left <= right && nums[right] >= 1) {
                right--;
            }
            
            if (left <= right) {
                int temp = nums[left];
                nums[left] = nums[right];
                nums[right] = temp;
                
                left++;
                right--;
            }
        }
        
        //重设left right
        left = 0;
        right = nums.length - 1;
        
        //和2比较
        while (left <= right) {
            while (left < right && nums[left] < 2) {
                left++;
            }
            
            while (left <= right && nums[right] >= 2) {
                right--;
            }
            
            if (left <= right) {
                int temp = nums[left];
                nums[left] = nums[right];
                nums[right] = temp;
                
                left++;
                right--;
            }
        }
        
    }
}