zoj 1944 Tree Recovery

/*zoj 1944    圖論典型題，由樹的前序走訪和中序遍曆求後序遍曆。這題跟zoj_1500可以做比較*/#include <iostream>#include <cstdio>#include <string>using namespace std;string ans,a,b;void build( int l1,int r1,int l2,int r2 ){    int i,len=0;    if( l1>r1 || l2>r2 )    return;    ans+=a[l1];    for( i=l2;i<=r2;i++,len++ )    {        if( a[l1]==b[i] )            break;    }    build( l1+len+1,r1,i+1,r2 );    build( l1+1,l1+len,l2,i-1 );}int main(){    while( cin>>a>>b )    {        int i;        ans="";        build( 0,a.size()-1,0,b.size()-1 );        for( i=ans.size()-1;i>=0;i-- )            cout<<ans[i];        cout<<endl;    }    return 0;}