网易2016 实习研发工程师 [编程题]寻找第K大 and leetcode 215. Kth Largest Element in an Array

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有一个整数数组，请你根据快速排序的思路，找出数组中第K大的数。

给定一个整数数组a,同时给定它的大小n和要找的K(K在1到n之间)，请返回第K大的数，保证答案存在。

测试样例：

[1,3,5,2,2],5,3

返回：2

note:

注意手写快排的时候：

        while(i < j) {
            while(j > i && a[j] > a[left]) j--;
            while(i < j && a[i] <= a[left]) i++;
            if(i < j) {
                swap(a[i],a[j]);
            }
        }

while(j > i && a[j] > a[left]) j--;
while(i < j && a[i] <= a[left]) i++;

这两个顺序不能反了，不然下标会错位一个

 1 class Finder {
 2 public:
 3     int findKth(vector<int> a, int n, int K) {
 4         // write code here
 5         return quickfind(a,0,n - 1,K);
 6     }
 7
 8     int quickfind(vector<int> &a,int left,int right,int K) {
 9         int i = left,j = right;
10         while(i < j) {
11             while(j > i && a[j] > a[left]) j--;
12             while(i < j && a[i] <= a[left]) i++;
13             if(i < j) {
14                 swap(a[i],a[j]);
15             }
16         }
17         swap(a[left],a[i]);
18         int dis = right - i + 1;
19         if(dis == K){
20             return a[i];
21         }
22         else if(K < dis) {
23             return quickfind(a,i + 1,right,K);
24         }
25         else{
26             return quickfind(a,left,i - 1 ,K - dis);
27         }
28     }
29 };

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leetcode:

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215. Kth Largest Element in an Array

QuestionEditorial Solution

My Submissions

• Total Accepted: 67793
• Total Submissions: 195182
• Difficulty: Medium

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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 1 class Solution {
 2 public:
 3     int findKthLargest(vector<int>& nums, int k) {
 4         return quickfind(nums,0,nums.size() - 1,k);
 5     }
 6
 7     int quickfind(vector<int> &a,int left,int right,int K) {
 8         int i = left,j = right;
 9         while(i < j) {
10             while(j > i && a[j] > a[left]) j--;
11             while(i < j && a[i] <= a[left]) i++;
12             if(i < j) {
13                 swap(a[i],a[j]);
14             }
15         }
16         swap(a[left],a[i]);
17         int dis = right - i + 1;
18         if(dis == K){
19             return a[i];
20         }
21         else if(K < dis) {
22             return quickfind(a,i + 1,right,K);
23         }
24         else{
25             return quickfind(a,left,i - 1 ,K - dis);
26         }
27     }
28 };