Day 54: HTML Code (The Mirror Effect)

📅 Date: Jan 17, 2026

🧠 Mood: Mirror Mode 🪞

🔥 Topic: DSA: Recursion with Strings (Reverse & Palindrome)

🔤 Strings & Recursion

How do you process a String using Recursion?
Think of it like a Two Pointer approach, but without a `while` loop.

• We process the corners (Start and End).
• Then we ask Recursion to handle the remaining inner part.

---

🔄 1. Reverse a String

Task: Convert "SAHIL" to "LIHAS".

Logic: Swap `s[start]` and `s[end]`. Then call function for `start+1` and `end-1`.

#include <iostream>
using namespace std;

// Pass by Reference (&str) so changes happen in original string
void reverse(string &str, int i, int j) {
    
    // 1. BASE CASE (Stop when pointers cross)
    if(i > j) {
        return;
    }

    // 2. PROCESSING (Swap characters)
    swap(str[i], str[j]);

    // 3. RECURSIVE CALL (Move inside)
    reverse(str, i+1, j-1);
}

int main() {
    string name = "SAHIL";
    reverse(name, 0, name.length()-1);
    cout << "Reversed: " << name << endl;
    return 0;
}

---

🪞 2. Check Palindrome

A Palindrome reads the same forwards and backwards (e.g., "MAM", "RACECAR").

Logic:
1. Check if first and last chars are same.
2. If YES, ask recursion to check the inner string.
3. If NO, return False immediately.

#include <iostream>
using namespace std;

bool checkPalindrome(string &str, int i, int j) {
    
    // 1. BASE CASE (If we cross middle, it's a Palindrome)
    if(i >= j) {
        return true;
    }

    // 2. CHECK (If corners don't match, fail immediately)
    if(str[i] != str[j]) {
        return false;
    }

    // 3. RECURSIVE CALL (Check remaining part)
    return checkPalindrome(str, i+1, j-1);
}

int main() {
    string s = "racecar";
    bool isPal = checkPalindrome(s, 0, s.length()-1);
    
    if(isPal) {
        cout << "It is a Palindrome ✅";
    } else {
        cout << "Not a Palindrome ❌";
    }
    return 0;
}

---

🚀 Day 54 Challenge

1. Write a recursive function to check if a string is a Binary Palindrome (e.g., "1001" is True).
2. Power Set (Subsequences): This is hard. Try to print all subsequences of string "abc" (Hint: Include/Exclude Pattern).
   We will cover this in Day 55, but try researching it today!

Next Up: Day 55 - The Hardest Part: Recursion with Subsequences (Include/Exclude).