leetcode75-- Sort Color

# 小郑之家~

### 题目

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]

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 a one-pass algorithm using only constant space?



### 解决

class Solution(object):
def sortColors(self, nums):
"""
:type nums: List[int]
:rtype: None Do not return anything, modify nums in-place instead.
"""

summ1 = 0
summ2 = 0
for x in nums:
if x==1:
summ1 += 1
summ2 += 2
elif x==2:
summ1 += 2
summ2 += 2

nums[:] = [0]*(len(nums)-(summ2-summ1)-(2*summ1-summ2)/2)+[1]*(summ2-summ1)+[2]*((2*summ1-summ2)/2)
# print(ret)
# nums[:] = ret



summ1 = y+2z; summ2 = 2y+2z 其中y是1的个数，z是2的个数.

class Solution(object):
def sortColors(self, nums):
"""
:type nums: List[int]
:rtype: None Do not return anything, modify nums in-place instead.
"""
ret = [0]*3

for x in nums:
ret[x] += 1

nums[:] = [0]*ret[0]+[1]*ret[1]+[2]*ret[2]



class Solution(object):
def sortColors(self, nums):
"""
:type nums: List[int]
:rtype: None Do not return anything, modify nums in-place instead.
"""

red, blue = 0, len(nums)-1
i = 0
while i<= blue:
if nums[i]==0:
nums[i], nums[red] = nums[red], nums[i]
red += 1
elif nums[i]==2:
nums[i], nums[blue] = nums[blue], nums[i]
blue -= 1
i -= 1  # 这里减去1的原因是blue的刚换过来一个，刚换过来的还没有处理.

i += 1