215. Kth Largest Element in an Array

Kth Largest Element in an Array

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example, Given [3,2,1,5,6,4] and k = 2, return 5.

Note: You may assume k is always valid, 1 ? k ? array's length.

Credits:Special thanks to @mithmatt for adding this problem and creating all test cases.

Solution

public class Solution {
   public int findKthLargest(int[] nums, int k) {
        if (nums == null || nums.length == 0) return -1;
        int size = nums.length;

        if (k < 1 || k > size) return -1;

        int start = 0;
        int end = size - 1;

        while (start < end) {
            int temp = partition(nums, start, end);
            if (temp == k - 1) return nums[temp];
            else if (temp < k - 1) start = temp + 1;
            else end = temp - 1;
        }

        return nums[k - 1];
    }

    private int partition(int[] nums, int start, int end) {
        int pivot = nums[start];

        int i = start + 1;
        int j = end;

        while (true) {
            while (i <= end && nums[i] > pivot) {
                i++;
            }

            while (j >= start && nums[j] <= pivot) {
                j--;
            }

            if (i >= j || i == end + 1 || j == start - 1) {
                break;
            }

            swap(nums, i, j);
        }

        if (j >= start) {
            swap(nums, start, j);
            return j;
        } else {
            return start;
        }
    }

    private void swap(int[] nums, int start, int end) {
        int temp = nums[start];
        nums[start] = nums[end];
        nums[end] = temp;
    }
}