Leetcode-215-Kth Largest Element in an Array

Leetcode-215-Kth Largest Element in an Array

215. Kth Largest Element in an Array

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• Total Accepted: 109821
• Total Submissions: 290143
• Difficulty: Medium
• Contributors: Admin

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.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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题解：

（1），二分的用法，真的是哪里用都很奇妙

class Solution {
public:
    int QuickSearch(vector<int>& nums, int k, int left, int right){
        int lf = left, rg = right, midval = nums[left + (right-left)/2]; 
        while(lf <= rg){
            if(nums[lf] > midval){
                lf++; 
            }else if(nums[rg] < midval){
                rg--; 
            }else{
                swap(nums, lf, rg); 
                lf++; 
                rg--; 
            }
        }
        if(lf < right && lf <= k){
            return QuickSearch(nums, k, lf, right); 
        }
        if(rg > left && rg >= k){
            return QuickSearch(nums, k, left, rg); 
        }
        return nums[k]; 
    }
    void swap(vector<int>& nums, int a, int b){
        int tmp = nums[a]; 
        nums[a] = nums[b]; 
        nums[b] = tmp; 
    }
    int findKthLargest(vector<int>& nums, int k) {
        return QuickSearch(nums, k-1, 0, nums.size()-1); 
    }
};