递归

逆波兰数

#include <algorithm>#include <cstring>#include <cctype>#include <cstdio>using namespace std;char str[1100];double Ans(){    scanf( "%s", str );    if( str[0]=='*' )        return Ans()*Ans();    else if( str[0]=='+' )        return Ans()+Ans();    else if( str[0]=='-' )        return Ans()-Ans();    else if( str[0]=='/' )        return Ans()/Ans();    else        return atof( str );}int main(){    printf( "%lf\n", Ans() );    return 0;}

全排列

#include <algorithm>#include <cstring>#include <cstdio>using namespace std;int used[1100];int a[1100];void A( int m ){    if( m==5 )    {        for( int i=1;i<=4;i++ )            printf( "%d ", a[i] );        printf( "\n" );        return ;    }   for( int i=1;i<=4;i++ )   {       if( used[i]==0 )       {           used[i]=1;            a[m]=i;            A( m+1 );            used[i]=0;       }   }}int main(){    int i, j;    int m=1;   A( m );   return 0;}