DSA interview question

Reverse the Words in a String (and Check a Palindrome)

Medium · Algorithms · Topics: string, two pointer

The problem

A voice assistant sometimes produces sentences with the words in the wrong order and messy spacing, like " hello world ". A clean-up step must reverse the order of the words, remove leading and trailing spaces, and leave exactly one space between words. The same text utilities module also needs a palindrome check that ignores punctuation and case, so that "A man, a plan, a canal: Panama" counts as a palindrome. Both are two-pointer string problems, and word reversal has a neat O(1)-extra-space trick: reverse the whole string, then reverse each word back.

Example: "the sky is blue" → "blue is sky the".

Function signature

string reverseWords(string s);        // single spaces, no leading or trailing spaces
bool isPalindrome(const string& s);   // letters and digits only, case-insensitive

Test cases

Input	Output	Explanation
`reverseWords("the sky is blue")`	`"blue is sky the"`
`reverseWords(" hello world ")`	`"world hello"`	Leading and trailing spaces removed
`reverseWords("a good example")`	`"example good a"`	Multiple spaces collapse to one
`reverseWords("single")`	`"single"`
`reverseWords(" ")`	`""`	Only spaces
`isPalindrome("A man, a plan, a canal: Panama")`	true	Ignoring punctuation and case
`isPalindrome("race a car")`	false
`isPalindrome(" ")`	true	No letters or digits: an empty string is a palindrome

(Confirm: words are separated by spaces only; the palindrome check considers letters and digits and ignores case.)

How interviewers phrase it

"Reverse the words in a string" (LeetCode 151), often with "can you do it in place with O(1) extra space?"
"Valid palindrome" (LeetCode 125), ignoring non-alphanumeric characters and case.

Clarifying questions to ask

Can there be leading, trailing or repeated spaces? How should they be handled?
Is modifying the input string allowed? (In C++ it's mutable; in Java/Python strings are immutable.)
For the palindrome: which characters count, and is it case-sensitive?

Follow-ups to expect

"How do you do it in O(1) extra space?"
"Why does reversing twice work?"
"What's the simpler approach?"
"How does the palindrome check handle punctuation?"
"Complexity?"

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