Reverse a String

🧩 Problem Statement

Write a function to reverse a string without using built-in functions like StringBuilder.reverse().

✍️ Function Signature

public String reverseString(String s)

---

🚀 Approaches

🧠 1. Two-Pointer Technique

Swap characters from both ends moving toward the center.

public String reverseString(String s) {
    char[] chars = s.toCharArray();
    int left = 0, right = chars.length - 1;
    while (left < right) {
        char temp = chars[left];
        chars[left] = chars[right];
        chars[right] = temp;
        left++;
        right--;
    }
    return new String(chars);
}

---

🔁 2. Using a Char Array and Iteration

Build the reversed string manually.

public String reverseString(String s) {
    char[] input = s.toCharArray();
    String result = "";
    for (int i = input.length - 1; i >= 0; i--) {
        result += input[i]; // Note: inefficient for large strings
    }
    return result;
}

---

🧱 3. Using a Character Stack (Simulating Recursion Stack)

Good for interview discussion on stack usage.

public String reverseString(String s) {
    Stack<Character> stack = new Stack<>();
    for (char c : s.toCharArray()) {
        stack.push(c);
    }
    StringBuilder reversed = new StringBuilder();
    while (!stack.isEmpty()) {
        reversed.append(stack.pop());
    }
    return reversed.toString();
}

---

🧮 4. Recursion

Elegant but less performant due to overhead.

public String reverseString(String s) {
    if (s == null || s.length() <= 1) {
        return s;
    }
    return reverseString(s.substring(1)) + s.charAt(0);
}

---

🧪 Test Cases

public class ReverseStringTest {
    public static void main(String[] args) {
        test("hello", "olleh");
        test("racecar", "racecar");
        test("12345", "54321");
        test("A", "A");
        test("", "");
    }

    private static void test(String input, String expected) {
        TwoSumSolver solver = new TwoSumSolver();
        String result = solver.reverseString(input);
        System.out.println("Input: " + input);
        System.out.println("Output: " + result);
        System.out.println("Expected: " + expected);
        System.out.println(result.equals(expected) ? "✅ Passed" : "❌ Failed");
        System.out.println("-----------------------------");
    }
}

---

📝 Notes

• Avoid += string concatenation for long inputs in interviews—mention using StringBuilder.
• Recursive solution may be less efficient due to call stack depth.
• Talk through edge cases like empty strings, single characters, and palindromes.

---

Happy coding! 🔄