#include<stdio.h>
#include<iostream>
using namespace std;

char s[10001],str[80];
int main()
{

#ifndef ONLINE_JUDGE
   freopen("input.txt", "rt", stdin);
   freopen("output.txt", "wt", stdout);
#endif

         int j = 0;
         memset(s,'\0',sizeof(s));
        while(gets(s))
        {
                int len = strlen(s);
                int i = 0;
                while(i<len)
                {
                        if(s[i]=='>')
                        {
                                j++;
                                if(j>79)j=0;
                        }
                        else if(s[i]=='<')
                        {
                                j--;
                                if(j<=0)j=0;
                        }
                        else if(s[i]!='\n' && s[i]!='\0')
                        {
                                str[j] = s[i];
                                j++;
                        }
                        i++;
                }

                memset(s,'\0',sizeof(s));
        }
for(int i = 0;i<80;i++)
printf("%c",str[i]);

return 0;
}

"at the beginning the e-screen contains 80 spaces"

I add in begin
memset(str,' ',sizeof(80));

I get WA#1 to former =(

Thank you!!!You help me!!!I got AC!!!=)