UVA10763 Foreign Exchange【map+数组】

Your non-profit organization (iCORE - international Confederation of Revolver Enthusiasts) coordinates a very successful foreign student exchange program. Over the last few years, demand has sky-rocketed and now you need assistance with your task.

  The program your organization runs works as follows: All candidates are asked for their original location and the location they would like to go to. The program works out only if every student has a suitable exchange partner. In other words, if a student wants to go from A to B, there must be another student who wants to go from B to A. This was an easy task when there were only about 50 candidates, however now there are up to 500000 candidates!

Input

The input file contains multiple cases. Each test case will consist of a line containing n – the number of candidates (1 ≤ n ≤ 500000), followed by n lines representing the exchange information for each candidate. Each of these lines will contain 2 integers, separated by a single space, representing the candidate's original location and the candidate’s target location respectively. Locations will be represented by nonnegative integer numbers. You may assume that no candidate will have his or her original location being the same as his or her target location as this would fall into the domestic exchange program. The input is terminated by a case where n = 0; this case should not be processed.

Output

For each test case, print ‘YES’ on a single line if there is a way for the exchange program to work out, otherwise print ‘NO’.

Sample Input

10

1 2

2 1

3 4

4 3

100 200

200 100

57 2

2 57

1 2

2 1

10

1 2

3 4

5 6

7 8

9 10

11 12

13 14

15 16

17 18

19 20

0

Sample Output

YES

NO

问题链接：UVA10763 Foreign Exchange

问题简述：（略）

问题分析：

  这里给出两种解法，一是使用map进行统计正和负统计，统计结果变为0则能够交换，不变为0则没法交换；二是使用数组进行模拟交换，又换回去了则说明能够交换。

程序说明：（略）

题记：（略）

参考链接：（略）

AC的C++语言程序如下：

/* UVA10763 Foreign Exchange */

#include 

using namespace std;

const int N = 5e5;
int ex[N + 1];

void init()
{
    for(int i=0; i

AC的C++语言程序如下：

/* UVA10763 Foreign Exchange */

#include 

using namespace std;

int main()
{
    int n;
    while(~scanf("%d", &n) && n) {
        map m;

        for(int i=1; i<=n; i++) {
            int a, b;
            scanf("%d%d", &a, &b);

            m[a]++;
            m[b]--;
        }

        bool ans = true;
        for(const auto &r : m)
            if(r.second) {
                ans = false;
                break;
            }

        printf("%s\n", ans ? "YES" : "NO");
    }

    return 0;
}