把仅含0，1，2的数组排序，要求只遍历一遍

思路：三指针

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: None Do not return anything, modify nums in-place instead.
        """
        l = 0
        r = len(nums)-1
        mid = 0
        while mid<=r:         
            if nums[mid] == 0:
                nums[mid],nums[l] = nums[l],nums[mid]
                l+=1
                mid+=1
            elif nums[mid]==1:
                mid+=1
            else:
                nums[r],nums[mid]=nums[mid],nums[r]
                r-=1
        return nums