# 75. Sort Colors 颜色分类

@TOC

## # 题目描述

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.

Example:

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

• 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?

## # 解题方法

### # 计数排序

``````from collections import Counter
class Solution(object):
def sortColors(self, nums):
"""
:type nums: List[int]
:rtype: void Do not return anything, modify nums in-place instead.
"""
count = Counter(nums)
for i in xrange(len(nums)):
if i < count[0]:
nums[i] = 0
elif i < count[0] + count[1]:
nums[i] = 1
else:
nums[i] = 2
``````

### # 双指针

i遇到0就换到左边去，遇到2就换到右边去，遇到1就跳过。

``````class Solution(object):
def sortColors(self, nums):
"""
:type nums: List[int]
:rtype: void Do not return anything, modify nums in-place instead.
"""
zero = 0
two = len(nums) - 1
i = 0
while i <= two:
if nums[i] == 0:
nums[zero], nums[i] = nums[i], nums[zero]
i += 1
zero += 1
elif nums[i] == 1:
i += 1
elif nums[i] == 2:
nums[two], nums[i] = nums[i], nums[two]
two -= 1

``````

## # 日期

2018 年 2 月 27 日 2019 年 1 月 5 日 —— 美好的周末又开始了