📘 Day 3 – Teacher's Day Challenge | Valid Palindrome Using Two Pointers

🗓️ Date: July 27, 2025
🔢 Problems Solved Today: 1
🎯 Focus Topic: Strings, Two Pointer Technique, Character Validation

🧩 Platform: Leetcode

---

🔗 Table of Contents

• Problem 1 – Valid Palindrome

• Daily Summary

---

🧩 Problem 1 – Valid Palindrome

📚 Difficulty: Easy
🧠 Concepts: Two Pointers, String Cleaning, Alphanumeric Check

---

📄 Problem Statement (Summary):
Given a string s, return true if it is a palindrome after:

• Converting to lowercase

• Removing all non-alphanumeric characters

Examples:

• Input: "A man, a plan, a canal: Panama" → Output: true

• Input: "race a car" → Output: false

• Input: " " → Output: true (Empty string after cleanup is a palindrome)

---

💡 Approach:

• Convert all characters to lowercase

• Use two pointers from start and end

• Skip non-alphanumeric characters using a helper function

• Compare characters and move inward

• If any mismatch found, return false; else return true

---

🧪 Code (C++):

class Solution {
public:
    bool isAlphaNum(char ch) {
        return (ch >= 'A' && ch <= 'Z' || 
                ch >= 'a' && ch <= 'z' || 
                ch >= '0' && ch <= '9');
    }

    bool isPalindrome(string s) {
        for (int i = 0; i < s.length(); i++) {
            s[i] = tolower(s[i]);
        }

        int st = 0, e = s.length() - 1;

        while (st <= e) {
            if (!isAlphaNum(s[st])) {
                st++;
                continue;
            }
            if (!isAlphaNum(s[e])) {
                e--;
                continue;
            }
            if (s[st] != s[e]) {
                return false;
            }
            st++, e--;
        }
        return true;
    }
};

🕒 Time Complexity: O(n)
📦 Space Complexity: O(1) (modifies in-place)

📸 Submission Screenshot:

---

✅ Key Takeaways:

• Lowercasing and character filtering is essential pre-processing

• Two pointer pattern is efficient for palindrome check

• Use helper method for clean alphanumeric validation

• Don’t forget empty string edge case – it’s valid

---

📈 Daily Summary

---

🏷️ Tags:
#leetcode #43DaysChallenge #palindrome #twopointers #cpp #strings #validPalindrome #dsa