nyoj 927 The partial sum problem（dfs）

 

描述

One day,Tom’s girlfriend give him an array A which contains N integers and asked him:Can you choose some integers from the N integers and the sum of them is equal to K. 
 

输入

There are multiple test cases.
Each test case contains three lines.The first line is an integer N(1≤N≤20),represents the array contains N integers. The second line contains N integers,the ith integer represents A[i](-10^8≤A[i]≤10^8).The third line contains an integer K(-10^8≤K≤10^8).

输出

If Tom can choose some integers from the array and their them is K,printf ”Of course,I can!”; other printf ”Sorry,I can’t!”.

样例输入

4
1 2 4 7
13
4
1 2 4 7
15

样例输出

Of course,I can!
Sorry,I can't!

两种方法：

第一种直接回溯dfs

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<math.h>
#include<algorithm>
#include<queue>
#include<set>
#include<bitset>
#include<map>
#include<vector>
#include<stdlib.h>
#include <stack>
using namespace std;
#define PI acos(-1.0)
#define max(a,b) (a) > (b) ? (a) : (b)
#define min(a,b) (a) < (b) ? (a) : (b)
#define ll long long
#define eps 1e-10
#define MOD 1000000007
#define N 26
#define inf 1e12
int n,m,flag;
int a[N];
int vis[N];
void dfs(int now,int num){
   if(num>=m){
         if(num==m){
            flag=1;
         }
         return;
   }
   for(int i=now;i<n;i++){
      if(!vis[i]){
         vis[i]=1;
         dfs(i+1,num+a[i]);
         if(flag){
            return;
         }
         vis[i]=0;
      }
   }
}
int main()
{
   while(scanf("%d",&n)==1){
      for(int i=0;i<n;i++){
         scanf("%d",&a[i]);
      }
      scanf("%d",&m);
      memset(vis,0,sizeof(vis));
      flag=0;
      dfs(0,0);
      if(flag){
         printf("Of course,I can!
");
      }else{
         printf("Sorry,I can't!
");
      }
   }
    return 0;
}

第二种类似01背包思想的dfs

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<math.h>
#include<algorithm>
#include<queue>
#include<set>
#include<bitset>
#include<map>
#include<vector>
#include<stdlib.h>
#include <stack>
using namespace std;
#define PI acos(-1.0)
#define max(a,b) (a) > (b) ? (a) : (b)
#define min(a,b) (a) < (b) ? (a) : (b)
#define ll long long
#define eps 1e-10
#define MOD 1000000007
#define N 26
#define inf 1e12
int n,m;
int a[N];
bool dfs(int cur,int num){
   if(num>=m){
      if(num==m){
         return true;
      }
      return false;
   }
   if(cur>=n) return false;
   if(dfs(cur+1,num+a[cur])) return true;
   return dfs(cur+1,num);

}
int main()
{
   while(scanf("%d",&n)==1){
      for(int i=0;i<n;i++){
         scanf("%d",&a[i]);
      }
      scanf("%d",&m);
      if(dfs(0,0)){
         printf("Of course,I can!
");
      }else{
         printf("Sorry,I can't!
");
      }
   }
    return 0;
}