POJ 题目Backward Digit Sums(next_permutation)

FJ and his cows enjoy playing a mental game. They write down the numbers from 1 to N (1 <= N <= 10) in a certain order and then sum adjacent numbers to produce a new list with one fewer number. They repeat this until only a single number is left. For example, one instance of the game (when N=4) might go like this: 

    3   1   2   4

      4   3   6

        7   9

         16

Behind FJ‘s back, the cows have started playing a more difficult game, in which they try to determine the starting sequence from only the final total and the number N. Unfortunately, the game is a bit above FJ‘s mental arithmetic capabilities. 

Write a program to help FJ play the game and keep up with the cows.

Line 1: Two space-separated integers: N and the final sum.

Line 1: An ordering of the integers 1..N that leads to the given sum. If there are multiple solutions, choose the one that is lexicographically least, i.e., that puts smaller numbers first.

4 16

3 1 2 4

Explanation of the sample: 

There are other possible sequences, such as 3 2 1 4, but 3 1 2 4 is the lexicographically smallest.

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<iostream>
using namespace std;
int a[15],b[15];
int main()
{
	int n,m;
	while(scanf("%d%d",&n,&m)!=EOF)
	{
		int i,j;
		for(i=1;i<=n;i++)
		{
			a[i]=i;
		}
		do{
			for(i=1;i<=n;i++)
				b[i]=a[i];
			for(i=1;i<=n;i++)
			{
				for(j=1;j<=n-i;j++)
					b[j]+=b[j+1];
			}
			if(b[1]==m)
				break;
			next_permutation(a+1,a+n+1);
		}while(1);
		for(i=1;i<=n;i++)
		{
			printf("%d%c",a[i],i==n?'\n':' ');
		}
	}
}