Kth Smallest Numbers in Unsorted Array

Find the kth smallest number in an unsorted integer array.

Example

Example 1:

Input: [3, 4, 1, 2, 5], k = 3
Output: 3

Example 2:

Input: [1, 1, 1], k = 2
Output: 1

Challenge

An O(nlogn) algorithm is acceptable, if you can do it in O(n), that would be great.

思路：quick select，模板一定要写熟！

public class Solution {
    /**
     * @param k: An integer
     * @param nums: An integer array
     * @return: kth smallest element
     */
    public int kthSmallest(int k, int[] nums) {
        if(nums == null || nums.length == 0) {
            return -1;
        }
        return findKth(nums, 0, nums.length - 1, k);
    }
    
    private int findKth(int[] A, int start, int end, int k) {
        if(start == end) {
            return A[start];
        }
        int mid = start + (end - start) / 2;
        int pivot = A[mid];
        int i = start, j = end;
        while(i <= j) {
            while(i <= j && A[i] < pivot) {
                i++;
            }
            while(i <= j && A[j] > pivot) {
                j--;
            }
            if(i <= j) {
                int temp = A[i];
                A[i] = A[j];
                A[j] = temp;
                i++;
                j--;
            }
        }
        
        if(start + k - 1 <= j) {
            return findKth(A, start, j, k);
        }
        if(start + k - 1 >= i) {
            return findKth(A, i, end, k - (i - start));
        }
        return A[j + 1];
    }
}