簡單題，顯然不能直接建立學校和學校之間的關係，因為一個學校可以和多個不同的學校交換，不是對映關係。

本人建立的對映是兩個學校之間交換學生數的對映

如 A 到 B 有 n 個學生 B 到 A 也有 n 個學生 那麼兩個學校之間就可以交換學生了，而這個關係是唯一的可以建立對映。

#include <bits/stdc++.h>

using namespace std;

struct student
{
    int from, to;
    bool operator < (const student &b)const{
        if(from == b.from)return to < b.to;
        return from < b.from;
    }
};
    student ss[500005];
int main()
{
    //freopen("in.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    int n;
    while(cin >> n && n){
        map<student, int> m;
         for (int i = 0; i < n; ++i)
        {
            cin >> ss[i].from >> ss[i].to;
            if(m[ss[i]]>=1)m[ss[i]]++;
            else m[ss[i]] = 1;
        }
        int ok = 1;
        for (int i = 0; i < n; ++i)
        {
            student temp;
            temp.to = ss[i].from;temp.from = ss[i].to;
            if(m[temp]!=m[ss[i]]){
                ok=0;
                break;
            }
        }
        if(ok)cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;

}

原題如下

 Your non-pro t organization (iCORE - international Confederation of Revolver Enthusiasts) coor-
dinates 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 le 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 can-
didate'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 loca-
tion 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