/in/foo.cc: In function 'int main()':
/in/foo.cc:9:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < password.length(); i++) { // 遍历每个字符
                     ^
/in/foo.cc:14:26: warning: array subscript is below array bounds [-Warray-bounds]
       cout << num[c-'a'+1];
                          ^