【USACO 2.3】Zero Sum（dfs）

按字典序输出所有在123..n之间插入'+','-',' '结果为0的表达式。.

http://train.usaco.org/usacoprob2?a=jUh88pMwCSQ&S=zerosum

/*
TASK:zerosum
LANG:C++
*/
#include<cstdio>
#include<string>
#include<algorithm>
using namespace std;
#define N 15
int n;
int k[N],cnt,s[]={,,-};
string ans[];
void dfs(int d){
    if(d>n){
        int sum=,c=;
        for(int i=;i<=n;i++){
            if(k[i]){
                sum+=c;
                c=i*k[i];
            }else{
                c=c*+(c>?i:-i);
            }
        }
        sum+=c;
        if(sum==){
            ans[++cnt]+="";
            for(int i=;i<=n;i++){
                ans[cnt]+=k[i]>?'+':k[i]<?'-':' ';
                ans[cnt]+=i+'';
            }
        }
        return;
    }
    for(int i=;i<=;i++){
        k[d]=s[i];
        dfs(d+);
    }
}
int main(){
    freopen("zerosum.in","r",stdin);
    freopen("zerosum.out","w",stdout);
    scanf("%d",&n);
    dfs();
    for(int i=;i<=cnt;i++){
        printf("%s\n",ans[i].c_str());
    }
}