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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。
[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:
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 };
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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原文地址：http://www.cnblogs.com/njczy2010/p/5726634.html
