215. 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.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5
Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4
Note:
You may assume k is always valid, 1 ≤ k ≤ array’s length.

快速排序的方法：時間複雜度O(n)

public int findKthLargest(int[] nums, int k) {
    k = nums.length - k;
    int left = 0;
    int right = nums.length - 1;
    while(left < right){
        int j = partition(nums,left,right);
        if(j == k) break;
        else if(j > k){
            right = j - 1;
        }else{
            left = j + 1;
        }
    }
    return nums[k];
}

private int partition(int[] nums, int lo, int hi) {
    int pivot = nums[lo];
    int i = lo + 1;
    int j = hi;
    while (lo < hi){
        while(i < hi && nums[i] <= pivot) i++; //和pivot相等的數應該看作小於pivot
        while(j > lo && nums[j] > pivot) j--;
        if(i >= j) break;
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
    int temp = nums[lo];
    nums[lo] = nums[j];
    nums[j] = temp;
    return j;
}