B - Sum It Up (数 与 和 )

Given a specified total t and a list of n integers, find all distinct sums using numbers from the list that add up to t. For example, if t=4, n=6, and the list is 4,3,2,2,1,14,3,2,2,1,1, then there are four different sums that equal 4: 4,3+1,2+2, and 2+1+1.(A number can be used within a sum as many times as it appears in the list, and a single number counts as a sum.) Your job is to solve this problem in general. 

InputThe input will contain one or more test cases, one per line. Each test case contains t, the total, followed by n, the number of integers in the list, followed by n integers x1,...,xn. If n=0 it signals the end of the input; otherwise, t will be a positive integer less than 1000, n will be an integer between 1 and 12(inclusive), and x1,...,xn will be positive integers less than 100. All numbers will be separated by exactly one space. The numbers in each list appear in nonincreasing order, and there may be repetitions. 
OutputFor each test case, first output a line containing 'Sums of', the total, and a colon. Then output each sum, one per line; if there are no sums, output the line 'NONE'. The numbers within each sum must appear in nonincreasing order. A number may be repeated in the sum as many times as it was repeated in the original list. The sums themselves must be sorted in decreasing order based on the numbers appearing in the sum. In other words, the sums must be sorted by their first number; sums with the same first number must be sorted by their second number; sums with the same first two numbers must be sorted by their third number; and so on. Within each test case, all sums must be distince; the same sum connot appear twice. 
Sample Input

4 6 4 3 2 2 1 1
5 3 2 1 1
400 12 50 50 50 50 50 50 25 25 25 25 25 25
0 0

Sample OutputSums of 4:

4
3+1
2+2
2+1+1
Sums of 5:
NONE
Sums of 400:
50+50+50+50+50+50+25+25+25+25
50+50+50+50+50+25+25+25+25+25+25

题目意思:给一个数N,和n个数,求后面多个数之和等于N的所有情况;

解法:用DFS找出所有可能性,注意去掉相同的情况

 

#include <stdio.h>  
#include <string.h>  
#include <algorithm>  
using namespace std;  
  
int n,len,a[20],b[20],cnt;  
  
int cmp(int a,int b)  
{  
    return a>b;  
}  
  
void dfs(int x,int posa,int sum,int posb)  
{  
    int i;  
    if(sum>n)  
        return;  
    if(sum == n)  
    {  
        cnt++;  
        for(i = 0; i<posb; i++)  
        {  
            if(i)  
                printf("+%d",b[i]);  
            else  
                printf("%d",b[i]);  
        }  
        printf("\n");  
    }  
    for(i = posa; i<len; i++)  
    {  
        b[posb] = a[i];  
        dfs(a[i],i+1,sum+a[i],posb+1);  
        while(i+1<len && a[i] == a[i+1])//搜索完毕后，若下一个搜索的数仍与当前相同，则跳过直至不相同    
            i++;  
    }  
}  
  
int main()  
{  
    int i;  
    while(~scanf("%d%d",&n,&len),n+len!=0)  
    {  
        for(i = 0; i<len; i++)  
            scanf("%d",&a[i]);  
        sort(a,a+len,cmp);  
        printf("Sums of %d:\n",n);  
        cnt = 0;  
        dfs(0,0,0,0);  
        if(!cnt)  
            printf("NONE\n");  
    }  
  
    return 0;  
}