搜索中的剪枝

1. 可行性剪枝

a)       

     

样例输入	样例输出
5 3 a e x y z	axy  axz  ayz  exy  exz  eyz

代码：

#include<cstring>

#include<cstdlib>

#include<cstdio>

#include<cmath>

#include<iostream>

#include<algorithm>

using namespace std;

int n,m,b[30],lasy,lasf;

char c[30],a[30];

void dfs(int now,int x,int nowy,int nowf)

{

      int t=0;

      if(nowy<1) t+=1-nowy;

      if(nowf<2) t+=2-nowf;

      if(t+now>m || (nowy<1 && x>lasy) || (nowf<2 && x>lasf)) return;

      if(now==m)

      {

           for(int i=1;i<=m;i++) printf("%c",a[i]);

           printf(" ");

           return;

      }

      if(x>n) return;

      if(b[x])

      {

           a[now+1]=c[x];

           dfs(now+1,x+1,nowy+1,nowf);

      }

      else

      {

           a[now+1]=c[x];

           dfs(now+1,x+1,nowy,nowf+1);

      }

     

      dfs(now,x+1,nowy,nowf);

}

int main()

{

      scanf("%d%d",&n,&m);

      for(int i=1;i<=n;i++)

      {

           char ch=getchar();

           while(ch<'a' || ch>'z') ch=getchar();

           c[i]=ch;

      }

      sort(c+1,c+n+1);

      for(int i=1;i<=n;i++)

           if(c[i]=='a' || c[i]=='e' || c[i]=='i' || c[i]=='o' || c[i]=='u') b[i]=1;

           else b[i]=0;

      for(int i=1;i<=n;i++)

           if(b[i]) lasy=i;

           else lasf=i;

      dfs(0,1,0,0);

      return 0;

}

1. 当前最优解剪枝

a)       

#include<cstring>

#include<cstdlib>

#include<cstdio>

#include<cmath>

#include<iostream>

#define N 110000

using namespace std;

double x[20],y[20],ans;

int n,cnt;

bool b[20];

double dis(int i,int j)

{

    double t1=x[i]-x[j],t2=y[i]-y[j];

    return sqrt(t1*t1+t2*t2);

}

void dfs(int x,double tmp)

{

    if(ans>0 && tmp>=ans) return;

    if(cnt==n)

    {

        ans=tmp;

        return;

    }

    for(int i=1;i<=n;i++)

    {

        if(b[i]) continue;

        cnt++;

        b[i]=1;

        dfs(i,tmp+dis(x,i));

        cnt--;

        b[i]=0;

    }

}

int main()

{

    scanf("%d",&n);

    ans=-1;

    for(int i=1;i<=n;i++) scanf("%lf%lf",&x[i],&y[i]);

    cnt=0;

    dfs(0,0);

    printf("%.2lf ",ans);

return 0;

}

1. 记忆化搜索

a)      Function：

                       i.              洛谷p1464

                     ii.              代码：