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 that nums[i] >= k.


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


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


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

Code - Java

public class Solution {
     * @param nums: The integer array you should partition
     * @param k:    As description
     *              return: The index after partition
    public int partitionArray(int[] nums, int k) {
        int j = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] < k) {
                int tmp = nums[i];
                nums[i] = nums[j];
                nums[j] = tmp;
        return j;