C++ runtime error: addition of unsigned offset? - c++

Question

I wrote the following to check if text is palindrome, I run it on leetcode and I am getting errors:
class Solution {
public:
bool isPalindrome(string s) {
int l=0,r=s.length()-1;
while(l<r)
{
while (!isalpha(s[r]))
{
--r;
}
while (!isalpha(s[l]))
{
++l;
}
if (tolower(s[r])!=tolower(s[l]))
return false;
--r;
++l;
}
return true;
}
};
Line 1061: Char 9: runtime error: addition of unsigned offset to
0x7ffc7cc10880 overflowed to 0x7ffc7cc1087f (basic_string.h) SUMMARY:
UndefinedBehaviorSanitizer: undefined-behavior
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/basic_string.h:1070:9
what's the problem with my code?

Answer 1

You're going out of bounds here:
while (!isalpha(s[r]))
and here
while (!isalpha(s[l]))
r can became negative and l can become >= s.length().
You should add some checks like
while (l < r && !isalpha(s[r]))
and
while (l < r && !isalpha(s[l]))
The same problem in this line
if (tolower(s[r])!=tolower(s[l]))
This should be
if (l < r && tolower(s[r])!=tolower(s[l]))
Different approach (C++20)
A different approach is to erase all non-alpha characters from s with
std::erase_if(s, [](char c) { return !isalpha(c); });
and remove the inner while loops.

Answer 2

I think you were very close to the solution. The pitfall here are that:
you are modifying the loop control variable more than once in the loop
(as consequence) you are using the loop control variable after changing their values without further checks.
The easy way to fix this kind of issue is to do one single action for every iteration. you can achieve this just using "else".
class Solution {
public:
bool isPalindrome(string s) {
int l=0,r=s.length()-1;
while(l<r)
{
if(!isalpha(s[r]))
{
--r;
}
else if(!isalpha(s[l]))
{
++l;
}
else if (tolower(s[r])!=tolower(s[l]))
{
return false;
}
else
{
--r;
++l;
}
}
return true;
}
};