View Compilation Error

0_0_20013121_19526.cpp: In function 'int solve(long long int)':
0_0_20013121_19526.cpp:25:20: error: 'strlen' was not declared in this scope
  int len = strlen(s); 
                    ^
0_0_20013121_19526.cpp: In function 'int main()':
0_0_20013121_19526.cpp:51:20: error: 'strlen' was not declared in this scope
  int len = strlen(s); 
                    ^