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题目大意：

   求第k个v出现的索引

解题思路：

  如果能构造出一个数据结构，使得data[v][k]就是第k个v出现的索引值即可求解。data[v]表示数v出现的索引数组,

data[v][k]表示第k个v出现的索引。

Though Rujia Liu usually sets hard problems for contests (for example, regional contests like Xi‘an 2006, Beijing 2007 and Wuhan 2009, or UVa OJ contests like Rujia Liu‘s Presents 1 and 2), he occasionally sets easy problem (for example, ‘the Coco-Cola Store‘ in UVa OJ), to encourage more people to solve his problems :D

Given an array, your task is to find the k-th occurrence (from left to right) of an integer v. To make the problem more difficult (and interesting!), you‘ll have to answer m such queries.

Input

There are several test cases. The first line of each test case contains two integers n, m(1<=n,m<=100,000), the number of elements in the array, and the number of queries. The next line contains n positive integers not larger than 1,000,000. Each of the following m lines contains two integer k and v (1<=k<=n, 1<=v<=1,000,000). The input is terminated by end-of-file (EOF). The size of input file does not exceed 5MB.

Output

For each query, print the 1-based location of the occurrence. If there is no such element, output 0 instead.

Sample Input

8 4
1 3 2 2 4 3 2 1
1 3
2 4
3 2
4 2

Output for the Sample Input

2
0
7
0

代码如下：

/*
 * UVA_11991.cpp
 *
 *  Created on: 2014年12月28日
 *      Author: Administrator
 */



#include <iostream>
#include <cstdio>
#include <map>
#include <vector>

using namespace std;

map<int ,vector<int> > mp;

int n,m;


int main(){
	while(scanf("%d%d",&n,&m)!=EOF){
		mp.clear();

		int i;
		for(i = 0 ; i < n ; ++i){
			int tmp;
			scanf("%d",&tmp);

			/**
			 * 返回map中键k的出现次数(对于map而言，由于一个key对应一个value，
			 * 因此返回只有0和1，因此可以用此函数判断k是否在map中)
			 */
			if(mp.count(tmp) == 0){//如果这个key值没有出现过
				mp[tmp] = vector<int> ();//那么创建一个新的vector。在这里要注意vector()的实例化
			}

			mp[tmp].push_back(i+1);
		}

		while(m--){
			int k,v;
			scanf("%d%d",&k,&v);

			if(mp.count(v) == 0 || mp[v].size() < k){//如果这个值不存在,或者是需要访问的位置超过了这个只出现的次数
				printf("0\n");//打印1
			}else{
				printf("%d\n",mp[v][k-1]);
			}
		}
	}

	return 0;
}

(DS 《算法入门经典》)UVA 11991 Easy Problem from Rujia Liu?(求第k个v出现的索引)

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原文地址：http://blog.csdn.net/hjd_love_zzt/article/details/42218653