UVA 11991 Easy Problem from Rujia Liu?

 1 #include <iostream>
 2 #include <cstring>
 3 #include <cstdio>
 4 #include <algorithm>
 5 #include <vector>
 6 #include <map>
 7 using namespace std;
 8 
 9 int main (){
10     int n,m;
11     map <int,vector<int> > x;
12     while (~scanf ("%d%d",&n,&m)){
13         x.clear() ;
14         for (int i=1;i<=n;i++){
15             int a;
16             scanf ("%d",&a);
17             if (!x.count (a))       //判断是否为空；
18                 x[a]=vector<int> {};
19             x[a].push_back (i);
20         }
21         while (m--){
22             int k,v;
23             scanf ("%d%d",&k,&v);//cout<<x[v][k-1]<<endl;
24             if (!x.count(v)||x[v].size()<k)
25                 printf ("0\n");
26             else printf ("%d\n",x[v][k-1]);
27         }
28     }
29     return 0;
30 }