利用 map 默认值。

#include <iostream>
#include <map>

using namespace std;

int main() {
    int count;
    while (cin >> count) {
        map<int, int> res;
        int key, value;
        for (int i = 0; i < count; ++i) {
            cin >> key >> value;
            int temp = res[key];
            res[key] = temp + value;
        }
        for (auto rec : res) {
            cout << rec.first << " " << rec.second << endl;
        }
    }
    return 0;
}