题解 | 特殊城市

#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <string>
#include <utility>
#include <vector>
using namespace std;

int main() {
    int n, ret = 0;
    cin >> n;
    string s, c;

    unordered_map<string, unordered_map<string, int>> hash;

    while (n--) {
        cin >> s  >> c;
        string prefix = s.substr(0, 2);
        if (prefix == c) {
            continue;
        }
        hash[prefix][c]++;
        ret += hash[c][prefix];
    }
    cout << ret;

    return 0;
}