POJ 3187 Backward Digit Sums

Description

输入n,sum，求1~n的数，如何排列之后，相邻两列相加，直到得出最后的结果等于sum，输出1~n的排列(杨辉三角)

http://poj.org/problem?id=3187

Code

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;

long long c[25][25];
void combine_init()
{
    memset(c, 0, sizeof(c));
    c[0][0] = 1;
    for(int i = 1; i <= 15; ++i)
    {
        c[i][0] = c[i][i] = 1;
        c[i][1] = i;
        for(int j = 1; j < i; ++j)
            if(c[i][j] == 0)
                c[i][j] = (c[i - 1][j] + c[i - 1][j - 1]);/// % mod
    }
}

int a[15];

int main()
{
    int n, num, sum;
    combine_init();
    while(cin >> n >> num)
    {
        for(int i = 0; i < n; ++i)
            a[i] = i + 1;
        do
        {
            sum = 0;
            for(int i = 0; i < n; ++i)
                sum += c[n - 1][i] * a[i];
            if(sum == num)
            {
                for(int i = 0; i < n; ++i)
                    cout << a[i] << ' ';
                cout << '\n';
                break;
            }
        }
        while(next_permutation(a, a + n));
    }
    return 0;
}