HDU 2616 Kill the monster【深搜】

Problem Description

There is a mountain near yifenfei’s hometown. On the mountain lived a big monster. As a hero in hometown, yifenfei wants to kill it.
Now we know yifenfei have n spells, and the monster have m HP, when HP <= 0 meaning monster be killed. Yifenfei’s spells have different effect if used in different time. now tell you each spells’s effects , expressed (A ,M). A show the spell can cost A HP to monster in the common time. M show that when the monster’s HP <= M, using this spell can get double effect.

Input

The input contains multiple test cases.
Each test case include, first two integers n, m (2<n<10, 1<m<10^7), express how many spells yifenfei has.
Next n line , each line express one spell. (Ai, Mi).(0<Ai,Mi<=m).

Output

For each test case output one integer that how many spells yifenfei should use at least. If yifenfei can not kill the monster output -1.

Sample Input

3 100
10 20
45 89
5  40

3 100
10 20
45 90
5 40

3 100
10 20
45 84
5 40

Sample Output

3
2
-1

分析：和N皇后问题一样，深搜找最优解。

code：

#include<stdio.h>
#include<string.h>
int a[11],b[11],vi[11];
int tot,n,v;
void dfs(int h,int k)
{
    int i;
    if(k>n)
        return;
    if(h<=0)
    {
        if(tot>k)
            tot=k;
        return;
    }
    for(i=0;i<n;i++)
    {
        if(!vi[i])
        {
            vi[i]=1;
            if(h<=b[i])
              dfs(h-2*a[i],k+1);
            if(h>b[i])
            dfs(h-a[i],k+1);
                vi[i]=0;
        }
    }
}
int main()
{
    int i;
    while(scanf("%d%d",&n,&v)!=EOF)
    {
        tot=20;
        for(i=0;i<n;i++)
            scanf("%d%d",&a[i],&b[i]);
        memset(vi,0,sizeof(vi));
            dfs(v,0);
        if(tot!=20)
            printf("%d\n",tot);
        else printf("-1\n");
    }
    return 0;
}