Palindrome
A palindrome reads the same forward and backward after applying the problem's normalization rules.
Start with this mental model
A palindrome reads the same forward and backward after applying the problem's normalization rules.
How it works, without unnecessary jargon
Two pointers can compare characters from both ends in O(n) time and O(1) extra space if normalization is done in-place/logically.
Always clarify whether spaces, punctuation and letter case should be ignored.
Read the algorithm as a sequence of decisions
Clarify Unicode/case/normalization assumptions.
Define indexes or prefix state.
Reuse previously learned match information when the algorithm allows it.
Read a tiny input by hand and write down the state after every important operation. For Palindrome, the goal is to make the invariant visible rather than to memorize code.
function isPalindrome(text) {
const s = text.toLowerCase().replace(/[^a-z0-9]/g, "");
let l = 0, r = s.length - 1;
while (l < r) {
if (s[l++] !== s[r--]) return false;
}
return true;
}Do not memorize the syntax. Follow the state that changes on each line and connect it to the invariant described above.
Varies by implementation / input constraints
Do not quote this complexity mechanically. Ask which operation or input dimension it describes, then derive the cost from the loops, recursion depth, data-structure operations and any preprocessing assumptions.
Good fit
- Use specialized matching algorithms when repeated or large-string searching makes naive scanning too expensive.
What usually goes wrong
- Ignoring Unicode semantics.
- Confusing substring with subsequence.
- Off-by-one prefix/index bugs.
A professional answer shape
Clarify input rules first; many wrong solutions solve a different normalization problem than the interviewer asked.
What problem does Palindrome solve?
A palindrome reads the same forward and backward after applying the problem's normalization rules.
What should I mention about complexity?
Its typical cost here is Varies by implementation / input constraints, but a complete answer should connect that cost to the representation, input constraints and the dominant operations.
What is the most common mistake?
Ignoring Unicode semantics.