算是一种递归完成的中序遍历，检查当前子树等价于检查左子树、根节点和右子树。

//例题8.4 二叉树
#include <iostream>
#include <algorithm>
using namespace std;
int sum;
void count_num(int m,int n){
    if(m>n)return;
    sum++;
    count_num(2*m,n);
    count_num(2*m+1,n);
    return ;
}

int main(){
    int m,n;
    while(scanf("%d%d",&m,&n)!=EOF){
        if(m==0)break;
        sum=0;
        count_num(m,n);
        cout<<sum<<endl;
    }
    return 0;
}

也可以不用全局变量：

//例题8.4 二叉树(思路一样但是更优雅的解法)
int count_num(int m,int n){
    if(m>n)return 0;
    return 1+count_num(2*m,n)+count_num(2*m+1,n);
}

int main(){
    int m,n;
    while(scanf("%d%d",&m,&n)!=EOF){
        if(m==0)break;
        printf("%d\n",count_num(m,n));
    }
    return 0;
}