LeetCode75：Sort Colors

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]

Follow up:

• 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?

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LeetCode：链接

设置两个头尾指针，头指针p0指向的位置是0该放置的位置，尾指针p2指向的位置是2该放置的位置。i用来遍历整个数组，碰到0把它和p0指向的数交换，碰到2把它和p2指向的数交换，碰到1继续向后遍历。

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        p0 = 0
        p2 = len(nums) - 1
        i = 0
        while i <= p2:
            if nums[i] == 2:
                nums[i], nums[p2] = nums[p2], nums[i]
                p2 -= 1
            elif nums[i] == 0:
                nums[i], nums[p0] = nums[p0], nums[i]
                p0 += 1
                i += 1
            else:
                i += 1