POJ 1564(HDU 1258 ZOJ 1711) Sum It Up（DFS）

题目链接：http://poj.org/problem?id=1564

题目大意：给定一个整数t，和n个元素组成的集合。求能否用该集合中的元素和表示该整数，如果可以输出所有可行解。1<=n<=12

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 Output

Sums 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

分析：

代码如下：

 1 # include<cstdio>
 2 # include<iostream>
 3 # include<algorithm>
 4 using namespace std;
 5 bool cmp(int a,int b){
 6     return a>b;
 7 }
 8 int a[15],t,n;
 9 bool flag;
10 int ans[15],len;
11 
12 void dfs(int sum,int mark){
13     int i;
14     if(sum == 0){
15         flag = true;
16         printf("%d",ans[0]);
17         for(i=1;i<len;i++)
18             printf("+%d",ans[i]);
19         printf("\n");
20         return;
21     }
22     if(sum<0 || mark>=n) return ;
23     for(i=mark; i<n; i++){
24         if(i==mark || a[i] != a[i-1]){
25             ans[len++] = a[i];
26             dfs(sum-a[i],i+1);
27             len--;
28         }
29     }
30 }
31 int main(){
32     int i;
33     while(scanf("%d%d",&t,&n) && n){
34         int temp = 0;
35         for(i=0;i<n;i++){
36             scanf("%d",&a[i]);
37             temp += a[i];
38         }
39         printf("Sums of %d:\n",t);
40         if(temp<t){
41             printf("NONE\n");
42             continue;
43         }
44         flag = false;
45         len = 0;
46         sort(a,a+n,cmp);
47         dfs(t,0);
48         if(!flag)
49             printf("NONE\n");
50         
51     }
52     return 0;
53 }