Median

 1 public class Solution {
 2     /**
 3      * @param nums: A list of integers.
 4      * @return: An integer denotes the middle number of the array.
 5      */
 6     public int median(int[] nums) {
 7         // write your code here
 8         if (nums == null || nums.length == 0) return -1;
 9         
10         int k = 0;
11         if (nums.length % 2 == 0) {
12             k = nums.length / 2 - 1;
13         } else {
14             k = nums.length / 2;
15         }
16         return kthElement(k, nums);
17     }
18     
19     public int kthElement(int k, int[] nums) {
20 
21         int start = 0;
22         int end = nums.length - 1;
23         
24         while (start <= end) {
25             int pivot = start;
26             for (int i = start; i <= end - 1; i++) {
27                 if (nums[i] < nums[end]) {
28                     swap(nums, pivot, i);
29                     pivot++;
30                 }
31             }
32             swap(nums, pivot, end);
33             
34             if (pivot == k) {
35                 return nums[pivot];
36             } else if (pivot < k) {
37                 start = pivot + 1;
38             } else {
39                 end = pivot - 1;
40             }
41         }
42         return -1;
43     }
44     
45     private void swap(int[] nums, int index1, int index2) {
46         int temp = nums[index1];
47         nums[index1] = nums[index2];
48         index[index2] = temp; 
49     }
50 }