[LeetCode 215] Kth Largest Element in an Array

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本文介绍了一种使用快速选择算法的变体来找到未排序数组中的第K大元素的方法。该算法通过分区操作实现了高效的查找过程，并且可以保证在1≤k≤数组长度的有效范围内始终返回正确的结果。

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.

Solution:

Introduction to Algorithm, there was one step in quickselect --- partition,

public int findKthLargest(int[] nums, int k) {
        return findK(nums, nums.length-k, 0, nums.length-1);
    }
    public int findK(int[] nums, int k, int s, int e) {
        if(s>=e) return nums[s];
        int m = partition(nums, s, e);
        if(m == k) return nums[m];
        else if(m<k) {
            return findK(nums, k, m+1, e);
        }else {
            return findK(nums, k, s, m-1);
        }
    }
    public int partition(int[] nums, int i, int j) {
        int pivot = nums[i];
        int m = i;
        int n = i+1;
        while(n<=j) {
            if(nums[n]<pivot){
                swap(nums, ++m, n);
            }
            n++;
        }
        swap(nums, i,m);
        return m;
    }
    public void swap(int[] nums, int a, int b) {
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }