POJ 3187 Backward Digit Sums

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

穷竭搜索 全排列 然后按规则求和 排列之前先按升序排序

这样可以保证第一个和为k的就是符合最小序列的结果

#include <iostream>
#include <stdio.h>
#include <algorithm>

using namespace std;

int sum(int a[], int n)
{
    int s[11];
    for (int i = 0; i < n; i++)
    {
        s[i] = a[i];
    }
    while (n != 1)
    {
        n--;
        for (int i = 0; i < n; i++)
        {
            s[i] += s[i+1];
        }
    }
    return s[0];
}
int main()
{
    int s,n;
    int a[11], calc[11];
    freopen("in.txt","r", stdin);
    while (~scanf("%d%d", &n, &s))
    {
        for(int i = 0; i < n; i++)
        {
            a[i] = i+1;
        }
        do
        {
            for (int i =0 ;i < n; i++)
            {
                calc[i] = a[i];
            }
            if ( sum(calc, n) == s) break;
        }while (next_permutation(a, a+n));
        for (int i =0 ; i < n; i++)
        {
            printf("%d ", calc[i]);
        }
        putchar('
');
    }
    return 0;
}