PAT1009

不难，只要注意0项不输出，从大到小输出即可。

#include<iostream>
#include<map>
#include<utility>
#include<iterator>
#include<iomanip>
using namespace std;

int main()
{
    int ain, bin;
    while(cin>>ain)
    {
        map<int, double>  a, b, c;
        for(int i=0; i < ain; ++i)
        {
            int key; double value;
            cin>>key>>value;
            a[key] = value;
        }
        cin>>bin;
        for(int i=0; i < bin; ++i)
        {
            int key; double value;
            cin>>key>>value;
            b[key] = value;
        }
        map<int,double>::iterator a_iter = a.begin();
        for(; a_iter != a.end(); ++a_iter)
        {
            map<int,double>::iterator b_iter = b.begin();
            for(; b_iter != b.end(); ++b_iter)
            {
                int k = a_iter->first + b_iter->first;
                c[k] += a_iter->second*b_iter->second;
            }
        }
        map<int, double>::reverse_iterator c_riter = c.rbegin();
        int num=0;
        for(; c_riter != c.rend(); ++c_riter)
            if(c_riter->second != 0)
                ++num;
        cout<<num;
        for(c_riter = c.rbegin(); c_riter != c.rend(); ++c_riter)
            if(c_riter->second != 0)
                cout<<fixed<<setprecision(1)<<" "<<c_riter->first<<" "<<c_riter->second;
        cout<<endl;
    }
    return 0;
}