Strings — Valid Palindrome II

solve(s) — s is made of lowercase letters. Return true if it is a palindrome, or
can become one after deleting at most one character.

Walk two pointers inward from both ends. While the characters match, keep going. On the
first mismatch you have exactly one chance: the answer is true if either
s[left+1..right] or s[left..right-1] is a plain palindrome (skip the left character, or
skip the right one). Checking both is O(n) overall.

Examples

solve("aba")    → true
solve("abca")   → true     // delete "c" (or "b")
solve("abc")    → false