Leetcode 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.

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

思路：
根据快速排序的partition，每次找到当前选取元素（第一个）在数组中的位置，如果这个位置等于nums.length - 1，就找到了要求的元素；如果小于，那么继续在右边找；如果大于，在左边继续找。

public int findKthLargest(int[] nums, int k) {
    if (k < 1 || nums == null) {
        return 0;
    }

    return getKth(nums.length - k +1, nums, 0, nums.length - 1);
}

public int getKth(int k, int[] nums, int start, int end) {

    int pivot = nums[end];

    int left = start;
    int right = end;

    while (true) {

        while (nums[left] < pivot && left < right) {
            left++;
        }

        while (nums[right] >= pivot && right > left) {
            right--;
        }

        if (left == right) {
            break;
        }

        swap(nums, left, right);
    }

    swap(nums, left, end);

    if (k == left + 1) {
        return pivot;
    } else if (k < left + 1) {
        return getKth(k, nums, start, left - 1);
    } else {
        return getKth(k, nums, left + 1, end);
    }
}

public void swap(int[] nums, int n1, int n2) {
    int tmp = nums[n1];
    nums[n1] = nums[n2];
    nums[n2] = tmp;
}