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：

# include<cstdio>
# include<iostream>
# include<algorithm>
using namespace std;
bool cmp(int a,int b){
    return a>b;
}
int a[15],t,n;
bool flag;
int ans[15],len;

void dfs(int sum,int mark){
    int i;
    if(sum == 0){
        flag = true;
        printf("%d",ans[0]);
        for(i=1;i<len;i++)
            printf("+%d",ans[i]);
        printf("
");
        return;
    }
    if(sum<0 || mark>=n) return ;
    for(i=mark; i<n; i++){
        if(i==mark || a[i] != a[i-1]){
            ans[len++] = a[i];
            dfs(sum-a[i],i+1);
            len--;
        }
    }
}
int main(){
    int i;
    while(scanf("%d%d",&t,&n) && n){
        int temp = 0;
        for(i=0;i<n;i++){
            scanf("%d",&a[i]);
            temp += a[i];
        }
        printf("Sums of %d:
",t);
        if(temp<t){
            printf("NONE
");
            continue;
        }
        flag = false;
        len = 0;
        sort(a,a+n,cmp);
        dfs(t,0);
        if(!flag)
            printf("NONE
");
        
    }
    return 0;
}