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;

        }

    }

}