1.applying non-zero offset to null pointer:

详细信息:
runtime error: applying non-zero offset 18446744073709551615 to null pointer (basic_string.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/basic_string.h:527:69

解决方案:
在函数中该返回string的地方返回了int。尤其注意是不是return "0"写成了return 0