Kth Largest Element in an Array

题目

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.

Note:
You may assume k is always valid, 1 ≤ k ≤ array's length.

答案

class Solution {
    public void swap(int[] nums, int left, int right) {
        int t = nums[left];
        nums[left] = nums[right];
        nums[right] = t;
    }
    
    public int partition(int[] nums, int left, int right, int k) {
        int pivotIndex = k;
        int pivot = nums[pivotIndex];
        int cut = left;
        swap(nums, pivotIndex, right);
        for(int i = left; i < right; i++) {
            if(nums[i] < pivot)
                swap(nums, i, cut++);
        }
        swap(nums, right, cut);
        return cut;
    }
    
    public int quickselect(int[] nums, int left, int right, int k) {
        
        int pivotIndex = partition(nums, left, right, k);
        if(pivotIndex == k)
            return nums[k];
        else if (k < pivotIndex)
            return quickselect(nums, left, pivotIndex - 1, k);
        else
            return quickselect(nums, pivotIndex + 1, right, k);
    }
    
    public int findKthLargest(int[] nums, int k) {
        return quickselect(nums, 0, nums.length - 1, nums.length - k);    
    }
}