75. Sort Colors (Medium)

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]

這道題第一個想法就是sort，當然題目說不要用sort，而且使用one-pass的方法，遍歷一遍且inplace解決 

思路：

使用index記錄0和2的位置，0位置初始化為0，2為位置初始化為len-1

遍歷到0或2就挪動交換到指示的位置

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        #1 sort
        #2 record index
        idx_0 = 0
        idx_2 = len(nums) - 1
        idx = 0
        while(idx < len(nums)):
            if nums[idx] == 0 and idx_0 < idx:
                nums[idx_0],nums[idx] = nums[idx],nums[idx_0]
                idx_0 += 1
            elif nums[idx] == 2 and idx_2 > idx:
                nums[idx_2],nums[idx] = nums[idx],nums[idx_2]
                idx_2 -= 1
            else:idx += 1