UVa 11991 一道简单题

https://vjudge.net/problem/UVA-11991

题意：
给出一个包含n个整数的数组，你需要回答若干询问。每次询问两个整数k和v，输出从左到右第k个v的下标。

思路：

把每个数字所出现的下标存储在vector中。

 #include<iostream>
 #include<string>
 #include<cstring>
 #include<algorithm>
 #include<vector>
 #include<map>
 using namespace std;

 const int maxn =  + ;

 int n, m;
 vector<int> s[maxn];

 int main()
 {
     //freopen("D:\\txt.txt", "r", stdin);
     while (cin >> n >> m)
     {
         int x;
         for (int i = ; i < maxn; i++)
             s[i].clear();
         for (int i = ; i <= n; i++)
         {
             cin >> x;
             s[x].push_back(i);
         }
         int k, v;
         for (int i = ; i < m; i++)
         {
             cin >> k >> v;
             if (k <= s[v].size())  cout << s[v][k - ] << endl;
             else cout << '' << endl;
         }
     }
 }

刘汝佳的做法更妙一些。

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
#include<vector>
#include<map>
using namespace std;

int n, m;
map <int, vector<int> > a;

int main()
{
    //freopen("D:\\txt.txt", "r", stdin);
    while (cin >> n >> m)
    {
        a.clear();
        int x;
        for (int i = ; i <= n; i++)
        {
            cin >> x;
            if (!a.count(x))   a[x] = vector<int>();
            a[x].push_back(i);
        }
        int k, v;
        for (int i = ; i < m; i++)
        {
            cin >> k >> v;
            if (k <= a[v].size())    cout << a[v][k - ] << endl;
            else cout << '' << endl;
        }
    }
}