View Compilation Error

0_0_38251042_11423.cpp: In function 'int main()':
0_0_38251042_11423.cpp:13:22: error: 'strlen' was not declared in this scope
 int len1 = strlen(a+1), len2 = strlen(b+1), len = max(len1, len2);
                      ^
0_0_38251042_11423.cpp:15:21: error: 'len' was not declared in this scope
 for(int j = 1; j <= len; j ++)
                     ^
0_0_38251042_11423.cpp:18:14: error: 'len2' was not declared in this scope
 int y = j <= len2 ? b[len2-j+1] - '0' : 0;
              ^
0_0_38251042_11423.cpp:24:21: error: 'len' was not declared in this scope
 for(int j = 1; j <= len; j ++)
                     ^