Partition Array

Problem

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 that nums[i] >= k.

Example

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

Note

You should do really partition in array nums instead of just counting the numbers of integers smaller than k.

If all elements in nums are smaller than k, then return nums.length

Challenge

Can you partition the array in-place and in O(n)?