題目：

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.

鏈接： http://leetcode.com/problems/sort-colors/

5/29/2017

0ms, 62%

設3個下標，分別標誌red, blue和當前遍歷到的。red, blue所指的地方是下一個可以判斷或者交換的位置。註意第10-14行也有交換的情況。並且第24行不需要index++

這種做法是通過中間的index來判斷swap和更新其他兩個下標的

 1 public class Solution {
 2     public void sortColors(int[] nums) {
 3         if (nums == null | nums.length == 0) return
 
;
 4         
 5         final int red = 0, white = 1, blue = 2;
 6         int redIndex = 0, index = 0, blueIndex = nums.length - 1;
 7         
 8         while (index <= blueIndex) {
 9             if (nums[index] == red) {
10                 int tmp = nums[redIndex];
11                 nums[redIndex] = red;
 
12                 nums[index] = tmp;
13                 index++;
14                 redIndex++;
15             } else if (nums[index] == white) {
16                 index++;
17             } else {
18                 if (nums[blueIndex] == blue) {
19                     blueIndex--;
20                 } else {
21                     int tmp = nums[blueIndex];
22                     nums[blueIndex] = blue;
23                     nums[index] = tmp;
24                     blueIndex--;
25                 }
26             }
27         }
28         return;
29     }
30 }

別人的回答

比較好的解釋

https://discuss.leetcode.com/topic/36832/sharing-c-solution-with-good-explanation

不需要swap的方法

算法特點，不使用if-else來判斷，並且blue的index也是從前向後走的。根據的是當前k(blueIndex)的值來判斷是否更新其他兩個下標和是否更新這兩個下標所指元素的值。

與swap方法的不同是，一個通過中間index判斷，一個通過最右邊index判斷。註意細節。

https://discuss.leetcode.com/topic/26181/ac-python-in-place-one-pass-solution-o-n-time-o-1-space-no-swap-no-count

 1 def sortColors(self, nums):
 2     i = j = 0
 3     for k in xrange(len(nums)):
 4         v = nums[k]
 5         nums[k] = 2
 6         if v < 2:
 7             nums[j] = 1
 8             j += 1
 9         if v == 0:
10             nums[i] = 0
11             i += 1

更多討論

https://discuss.leetcode.com/category/83/sort-colors

75. Sort Colors