ReverseString.java

package com.thealgorithms.strings;

import java.util.Stack;

/**
 * Reverse String using different version
 */
public final class ReverseString {
    private ReverseString() {
    }

    /**
     * easiest way to reverses the string str and returns it
     *
     * @param str string to be reversed
     * @return reversed string
     */
    public static String reverse(String str) {
        return new StringBuilder(str).reverse().toString();
    }

    /**
     * second way to reverses the string str and returns it
     *
     * @param str string to be reversed
     * @return reversed string
     */
    public static String reverse2(String str) {
        if (str == null || str.isEmpty()) {
            return str;
        }

        char[] value = str.toCharArray();
        for (int i = 0, j = str.length() - 1; i < j; i++, j--) {
            char temp = value[i];
            value[i] = value[j];
            value[j] = temp;
        }
        return new String(value);
    }

    /**
     * Reverse version 3 the given string using a StringBuilder.
     * This method converts the string to a character array,
     * iterates through it in reverse order, and appends each character
     * to a StringBuilder.
     *
     * @param string The input string to be reversed.
     * @return The reversed string.
     */
    public static String reverse3(String string) {
        if (string.isEmpty()) {
            return string;
        }
        char[] chars = string.toCharArray();
        StringBuilder sb = new StringBuilder();
        for (int i = string.length() - 1; i >= 0; i--) {
            sb.append(chars[i]);
        }
        return sb.toString();
    }
    /**
     * Reverses the given string using a stack.
     * This method uses a stack to reverse the characters of the string.
     * * @param str The input string to be reversed.
     * @return The reversed string.
     */
    public static String reverseStringUsingStack(String str) {
        // Check if the input string is null
        if (str == null) {
            throw new IllegalArgumentException("Input string cannot be null");
        }
        Stack<Character> stack = new Stack<>();
        StringBuilder reversedString = new StringBuilder();
        // Check if the input string is empty
        if (str.isEmpty()) {
            return str;
        }
        // Push each character of the string onto the stack
        for (char ch : str.toCharArray()) {
            stack.push(ch);
        }
        // Pop each character from the stack and append to the StringBuilder
        while (!stack.isEmpty()) {
            reversedString.append(stack.pop());
        }
        return reversedString.toString();
    }

    /**
     * Reverse the String using Recursion
     * @param str string to be reversed
     * @return reversed string
     */
    public static String reverseStringUsingRecursion(String str) {
        if (str.isEmpty()) {
            return str;
        } else {
            return reverseStringUsingRecursion(str.substring(1)) + str.charAt(0);
        }
    }
}