为什么会产生段错位呢？好心人帮忙回答一下。

#include<iostream>
#include<string>
using namespace std;
int main()
{
    string A = "0123456789ABCDEFGHIJKLMNOPQRSTUV";
    string  str;
    while (cin >> str)
    {
        string str1=" ";
             int t=1,num=0;
             int i=str.length()-1;
                for(int j=0;i-j>4;j++)
                {
                    num = num+(str[i-j]-'0')* t;
                    t=t*2;
                    }
                str1=A[num];
                num=0;
                  t=1;
              for(int a=0;a<=4;a++)
                     {
                    num += (str[a]-'0') * t;
                    t=2*t;
                    }
            str1+=A[num];
     cout << str1<<endl;
    }#学习路径#