option=com_onlinejudge&Itemid=8&page=show_problem&problem=3142" target="_blank" style="">题目链接：uva 11991 - Easy Problem from Rujia Liu?

题目大意：给出一个包括n个整数的数组，你须要回答若干询问，每次询问两个整数k和v，输出从左到右第k个v的下标

解题思路：用map映射一个vector，相应即为map<int>即为一个可变长的数组。读取数组的时候将相应值放入就可以。

#include <cstdio>

#include <cstring>

#include <map>

#include <vector>

#include <algorithm>

using namespace std;

map<int, vector<int> > g;

int main () {

    int N, M, x, y;

    while (scanf("%d%d", &N, &M) == 2) {

        g.clear();

        for (int i = 1; i <= N; i++) {

            scanf("%d", &x);

            if (!g.count(x))

                g[x] = vector<int>();

            g[x].push_back(i);

        }

        for (int i = 0; i < M; i++) {

            scanf("%d%d", &x, &y);

            if (!g.count(y) || g[y].size() < x)

                printf("0\n");

            else

                printf("%d\n", g[y][x-1]);

        }

    }

    return 0;

}