【PAT (Advanced Level) Practice】1009 Product of Polynomials (25 分)

This time, you are supposed to find $A×B$ where $A$ and $B$ are two polynomials.

Input Specification:
Each input file contains one test case. Each case occupies $2$ lines, and each line contains the information of a polynomial:

$K ~N_1~ a_{N_1} ~ N2 ~ a_{N_2} ~ ... ~ N_K ~ a_{N_K}$

where $K$ is the number of nonzero terms in the polynomial, $N_i$ and $a_{N_i}(i=1,2,⋯,K)$ are the exponents and coefficients, respectively. It is given that $1≤K≤10，0≤N_K<⋯<N_2<N_1≤1000$.

Output Specification:
For each test case you should output the product of $A$ and $B$ in one line, with the same format as the input. Notice that there must be NO extra space at the end of each line. Please be accurate up to $1$ decimal place.

Sample Input:

2 1 2.4 0 3.2
2 2 1.5 1 0.5

Sample Output:

3 3 3.6 2 6.0 1 1.6

---

#include<iostream>

using namespace std;

const int N = 1010;

double a[N], b[N], c[N*2];

void input(double a[]){
    
    int k;
    cin >> k;
    while(k--){
        
        int n; double v;
        cin >> n >> v;
        a[n] = v;
    }
}

int main(){
    
    input(a);
    input(b);
    
    for(int i = 0; i < N; i++)
        for(int j = 0; j < N; j++)
            c[i + j] += a[i] * b[j];
    
    int k = 0;
    for(int i = 0; i < N * 2; i++)
        if(c[i])
            k++;
    
    cout << k;
    for(int i = 2 * N - 1; i >= 0; i--)
        if(c[i])
            printf(" %d %.1lf", i, c[i]);
    
    return 0;
}