acdream 1726 A Math game（部分和）

A Math game

Time Limit: 2000/1000MS (Java/Others)    Memory Limit: 256000/128000KB (Java/Others)

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Problem Description

Recently, Losanto find an interesting Math game. The rule is simple: Tell you a number H, and you can choose some numbers from a set {a[1],a[2],......,a[n]}.If the sum of the number you choose is H, then you win. Losanto just want to know whether he can win the game.

Input

There are several cases.
In each case, there are two numbers in the first line n (the size of the set) and H. The second line has n numbers {a[1],a[2],......,a[n]}.0<n<=40, 0<=H<10^9, 0<=a[i]<10^9,All the numbers are integers.

Output

If Losanto could win the game, output "Yes" in a line. Else output "No" in a line.

Sample Input

10 872 3 4 5 7 9 10 11 12 1310 382 3 4 5 7 9 10 11 12 13

Sample Output

NoYes

思路：求部分和，用DFS，对于每一个数，选或者不选， 需要剪枝。

#include<cstdio>using namespace std;const int maxn=50;int num[maxn], n, m;long  long sum[maxn];int DFS(long long s, int pos){    if(s==m)        return 1;    if(sum[n]-sum[pos-1]+s<m || pos>n || s>m)        return 0;    if(DFS(s+num[pos], pos+1))        return 1;    if(DFS(s, pos+1))        return 1;    return 0;}int main(){    while(~scanf("%d%d", &n, &m))    {        for(int i=1; i<=n; i++)        {            scanf("%d", num+i);            sum[i]=sum[i-1]+num[i];        }        if(DFS(0, 1))            printf("Yes\n");        else            printf("No\n");    }    return 0;}