Partition Array

Given an array nums of integers and an int k, partition the array (i.e move the elements in "nums") such that:

All elements < k are moved to the left

All elements >= k are moved to the right

Return the partitioning index, i.e the first index i nums[i] >= k.

Example

If nums = [3,2,2,1] and k=2, a valid answer is 1.

Solution

Partition value is the key to quick sort and this question.

This question is classic two pointers question.

the inner while is the slide operation

it's very important left value < k and right value >=k ?

class Solution:
    """
    @param nums: The integer array you should partition
    @param k: As description
    @return: The index after partition
    """
    def partitionArray(self, nums, k):
        start, end = 0, len(nums) - 1
        while start <= end:
            while start <= end and nums[start] < k:
                start += 1
            while start <= end and nums[end] >= k:
                end -= 1
            if start <= end:
                nums[start], nums[end] = nums[end], nums[start]
                start += 1
                end -= 1
        return start