View Compilation Error

0_0_16754768_29070.cpp: In function 'int main()':
0_0_16754768_29070.cpp:19:27: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
    strcat(str1,str3[len1/2]);
                           ^
In file included from 0_0_16754768_29070.cpp:2:0:
GCC4.9.2/x86_64-w64-mingw32/include/string.h:52:18: note: initializing argument 2 of 'char* strcat(char*, const char*)'
   char * __cdecl strcat(char * __restrict__ _Dest,const char * __restrict__ _Source);
                  ^