View Compilation Error

0_0_20935257_30530.cpp: In function 'bool ok(char*)':
0_0_20935257_30530.cpp:20:16: error: 'strlen' was not declared in this scope
  for(i=strlen(s)-1;i>=0;i--)
                ^
0_0_20935257_30530.cpp: In function 'void bfs()':
0_0_20935257_30530.cpp:63:25: error: 'strlen' was not declared in this scope
    if(ct.p+1<strlen(ct.s))
                         ^
0_0_20935257_30530.cpp:91:25: error: 'strlen' was not declared in this scope
    if(ct.p+1<strlen(ct.s))
                         ^