View Compilation Error

0_0_22426878_1352.cpp: In function 'int main()':
0_0_22426878_1352.cpp:11:29: error: 'strlen' was not declared in this scope
   int length_n1 = strlen(n_1), length_n2 = strlen(n_2);
                             ^
0_0_22426878_1352.cpp:91:24: error: 'length_n2' was not declared in this scope
    for (int i = 1; i < length_n2; ++i) {
                        ^
0_0_22426878_1352.cpp:114:24: error: 'length_n2' was not declared in this scope
    for (int i = 1; i < length_n2; ++i) {
                        ^
0_0_22426878_1352.cpp:137:24: error: 'length_n2' was not declared in this scope
    for (int i = 0; i < length_n2; ++i) {
                        ^