小弟在编写一个C程序的时候，发现执行的时候，总是出错，错误是“xx.exe已停止工作”具体程序如下：
#include <stdio.h>
#include <string.h>

void expand(char* s1,char* s2)
{
     int i,j;
     char temp;
     
     for(i = 0;i < strlen(s1) - 1; )
     {
           if( (s1[i] == '-') && s1[i+2] != '-')
           {
               temp = s1[++i] ;
               for( ; s2[j++] != temp; )
               {
                    s2[j] = s2[j-1] + 1;
               }
               i = i + 2;
           }
           else if( (s1[i] == '-') && s1[i+2] == '-')
           {
               temp = s1[i+3] ;
               for( ; s2[j++] != temp; )
               {
                    s2[j] = s2[j-1] - 1;
               }
               i = i + 4;
           }
           else
           {
               s2[j++] = s1[i++];
           }
     }
}

main()
{
      char s1[] = "a-z0-9";
      char s2[100];
      
      expand(s1,s2);
      
      printf("Now S1 is %s and S2 is %s",s1,s2);
      
}
所用的编译环境是DEV-Cpp(4.9.9.2），操作系统是win7

s2[j++] = s1[i++];
j还没初始化就执行了所以出错。