The sword refers to offer-the smallest k number

 

Topic description

 

Enter n integers and find the smallest K number among them. For example, if you enter 8 numbers 4,5,1,6,2,7,3,8, the smallest 4 numbers are 1,2,3,4.

 

Problem solving ideas

 

Referring to the idea of ​​divide and conquer in quick sort, first call the Partition function to find the subscript m of the divide and conquer point. If m is equal to k, the number of m before m is the solution; if m is less than k, continue to find the right side of m. Divide and conquer point; if m is greater than k, continue to find the divide and conquer point on the left of m until m and k are equal.

 

code

 

 1 class Solution {
 2 public:
 3     vector<int> GetLeastNumbers_Solution(vector<int> &input, int k) {
 4         vector<int> v;
 5         int l=input.size();
 6         if(input.empty()||k<1||k>l)
 7             return v;
 8         if(k==l)
 9             return input;
10         int m=Partition(input,0,l-1);
11         while(m!=k){
12             if(m<k)
13                 m=Partition(input,m+1,l-1);
14             else
15                 m=Partition(input,0,m-1);
16         }
17         for(int i=0;i<m;i++)
18             v.push_back(input[i]);
19         return v;
20     }
21     int Partition(vector<int> &a, int f, int l) {
22         int i = f - 1;
23         for (int j = f; j < l; j++)
24         {
25             if (a[j] <= a[l]) {
26                 i++;
27                 swap(a[i], a[j]);
28             }
29         }
30         i++;
31         swap(a[i], a[l]);
32         return i;
33     }
34 };