75.颜色分类

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        head, now, tail = 0, 0, len(nums)-1
        while now <= tail:
            if nums[now] == 0:
                nums[now], nums[head] = nums[head], nums[now]
                now, head= now+1, head+1
            elif nums[now] == 2:
                nums[now], nums[tail] = nums[tail], nums[now]
                tail -= 1
            else:
                now += 1