Kth Largest Element in an Array

Find K-th largest element in an array.

Notice

You can swap elements in the array

Example

In array [9,3,2,4,8], the 3rd largest element is 4.

In array [1,2,3,4,5], the 1st largest element is 5, 2nd largest element is 4, 3rd largest element is 3 and etc.

分析：

使用partion把array分成两组，然后看中间那个数在哪个位置。然后再确定是继续在左半部分找还是在右半部分找。

 public class Solution {
     public int findKthLargest(int[] nums, int k) {
         if (nums == null || nums.length ==  || k > nums.length) return -;
         int start = , end = nums.length - ;
         while (start <= end) {
             int p = partition(nums, start, end);
             if (p == nums.length - k) {
                 return nums[p];
             } else if (p < nums.length - k) {
                 start = p + ;
             } else {
                 end = p - ;
             }
         }
         return -;
     }

     private int partition(int[] nums, int start, int end) {
         int p = start;
         for (int i = start; i <= end - ; i++) {
             if (nums[i] < nums[end]) {
                 swap(nums, p, i);
                 p++;
             }
         }
         swap(nums, p, end);
         return p;
     }

     private void swap(int[] nums, int i, int j) {
         int temp = nums[i];
         nums[i] = nums[j];
         nums[j] = temp;
     }
 }

 public void print3largest(int[] arr) {
         if (arr == null || arr.length < ) {
             System.out.print(" Invalid Input ");
             return;
         }

         int i, first, second, third;

         third = first = second = Integer.MIN_VALUE;
         for (i = ; i < arr.length; i++) {
             if (arr[i] > first) {
                 third = second;
                 second = first;
                 first = arr[i];
             } else if (arr[i] > second) {
                 third = second;
                 second = arr[i];
             } else if (arr[i] > third) {
                 third = arr[i];
             }
         }

         System.out.println("Three largest elements are " + first + " " + second + " " + third);
     }

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