31. 数组划分

class Solution:
    """
    @param nums: The integer array you should partition
    @param k: An integer
    @return: The index after partition
    """
    ‘‘‘
    大致思路：背向型双指针
    1.小于的话丢左边，大于的话丢右边，最终left就是所要放置的位置
    ‘‘‘
    def partitionArray(self, nums, k):
        #初始化
        length = len(nums)
        left, right = 0, length - 1
        index = 0 

        #left指针指向比k小的数，right指针指向的是比k大的数,第三个指针默认走
        while index <= right:
            if (nums[index] < k):
                nums[left], nums[index] = nums[index], nums[left]
                left += 1
                index += 1
            elif (nums[index] > k):
                nums[right], nums[index] = nums[index], nums[right]
                right -= 1
            else:
                index += 1
                continue
        
        return left