Longest Palindrome asks for the length of the longest palindrome you can build from a set of characters. Don’t confuse it with Longest Palindromic Substring — this one is about construction, not search. It tests whether you understand palindrome structure and can derive a greedy rule from first principles.

---

The Problem

Given a string s, return the length of the longest palindrome that can be built with its characters. Letters are case-sensitive.

Example

Input: s = "abccccdd" Output: 7 — one longest palindrome is "dccaccd" (2 d’s, 4 c’s, 1 a).

Already comfortable with the solution? Practice it in a mock interview →

---

What Interviewers Are Actually Testing

Skill	What Interviewers Observe
Palindrome structure	Do you know palindromes are built from pairs + at most one center?
Greedy reasoning	Can you derive the rule from first principles?
Case sensitivity	Do you treat ‘A’ and ‘a’ as different?

---

Step 1: Clarifying Questions

1. Case-sensitive? Yes — 'A' and 'a' are different.
2. Can we reorder characters? Yes — we’re building, not finding in-place.

---

Step 2: The Key Insight

A palindrome is made of pairs plus at most one center character. Each character with an even frequency contributes its full count. Each odd-frequency character contributes count - 1. If any character has an odd frequency, add 1 for the center.

---

Python Implementation

from collections import Counter

def longestPalindrome(s: str) -> int:
    freq = Counter(s)
    length = 0
    has_odd = False

    for count in freq.values():
        length += count // 2 * 2
        if count % 2 == 1:
            has_odd = True

    return length + (1 if has_odd else 0)

---

Time & Space Complexity

Aspect	Complexity
Time	O(n)
Space	O(k) — k unique characters

---

Common Interview Mistakes

1. Confusing with Longest Palindromic Substring. This builds a palindrome from characters; that finds one within a string.
2. Forgetting case sensitivity. 'A' and 'a' are separate entries.
3. Not adding the center +1. Forgetting the optional center gives length - 1.

---

What a Strong Candidate Sounds Like

“A palindrome consists of pairs plus an optional center. For each character, I use count // 2 * 2 — the even portion. If any character had an odd count, I place one in the center. The answer is the sum of all even portions plus 1 if any odd count exists.”

---

Related Problems to Practice

• Valid Palindrome — Two-pointer palindrome check.
• Longest Palindromic Substring — Finding palindromes in-place.
• Valid Anagram — Same frequency-counting pattern.

---

Practice This in a Mock Interview

Start a mock interview for Longest Palindrome on Intervu.

---

Further reading:

• How to Prepare for a Coding Interview in 2026, the complete roadmap
• Why LeetCode alone isn’t enough, and what to practice instead
• Practice any LeetCode problem as a live mock interview
• The Grind 75 Pathway, a structured study plan with AI practice links
• Browse all walkthroughs on GitHub