荷兰国旗问题，快速选择，partition，three way partition 快速选择的题，好题就多看几遍

1.荷兰国旗问题 【0，0，1，1，2，2，2】
题目参考链接

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        '''
        荷兰三色旗问题解
        '''
        # 对于所有 idx < p0 : nums[idx < p0] = 0
        # curr是当前考虑元素的下标
        p0 = curr = 0
        # 对于所有 idx > p2 : nums[idx > p2] = 2
        p2 = len(nums) - 1
        
        while curr <= p2:
            if nums[curr] == 0:
                nums[p0], nums[curr] = nums[curr], nums[p0]
                p0 +=1
                curr +=1
            elif nums[curr] == 2:
                nums[p2], nums[curr] = nums[curr], nums[p2]
                p2 +=1
            else:
                curr +=1

2.摆动排序Ⅱ
题目参考链接

class Solution:
    def wiggleSort(self, nums):
        n = len(nums)
        if n < 2:return nums
        mid = (0 + n-1) // 2 # 中位数索引
        # 快速排序中的一次划分
        def partition(begin,end):
            left,right = begin,end
            while left < right:
                while left < right and nums[left] < nums[right]:
                    right -= 1
                if left < right:
                    nums[left],nums[right] = nums[right],nums[left]
                    left += 1
                while left < right and nums[left] < nums[right]:
                    left += 1
                if left < right:
                    nums[left],nums[right] = nums[right],nums[left]
                    right -= 1
            return left
        # 找到中位数对应的数值
        left,right = 0, n-1
        while True:
            pivot = partition(left,right)
            if pivot == mid:
                break
            elif pivot > mid:
                right = pivot - 1
            else:
                left = pivot + 1
        # 三路划分(荷兰旗)
        midNum = nums[mid]
        left,curr,right = 0, 0, n-1
        while curr <= right:
            if nums[curr] < midNum:
                nums[left],nums[curr] = nums[curr],nums[left]
                left += 1
                curr += 1
            elif nums[curr] > midNum:
                nums[curr],nums[right] = nums[right],nums[curr]
                right -= 1
            else:
                curr += 1
 	# 交叉合并
        small,big ,_nums = mid,n-1,nums[:]
        for i in range(n):
            if i%2 == 0:
                nums[i] = _nums[small]
                small -= 1
            else:#big
                nums[i] = _nums[big]
                big -= 1