Question 1: How to reverse a string in java? Can you write a program without using any java inbuilt methods?

 Solution: There are many ways to do it, some of them are:

• Using for loop
• Using recursion
• Using StringBuffer

Using for loop

1. Declare empty String reverse. This will contain our final reversed string.
2. Iterate over an array using for loop from last index to 0th index
3. Add character to String reverse while iterating.

package org.cloud.techtwitter.blogspot;
 
public class ReverseStringForMain {
    public static void main(String[] args) {
        String blogName = "techTwitter";
        String reverse = "";
        for (int i = blogName.length() - 1; i >= 0; i--) {
            reverse = reverse + blogName.charAt(i);
        }
        System.out.println("Reverse of cloud.techtwitter is: " + reverse);
    }
}

Using recursion

We can also use recursion to reverse a String in java

package org.cloud.techtwitter.blogspot;
 
public class ReverseStringRecursive {
    public static void main(String[] args) {
        ReverseStringRecursive rsr = new ReverseStringRecursive();
        String blogName = "techTwitter";
        String reverse = rsr.recursiveReverse(blogName);
        System.out.println("Reverse of cloud.techtwitter is:" + reverse);
    }
 
    public String recursiveReverse(String orig) {
        if (orig.length() == 1)
            return orig;
        else
            return orig.charAt(orig.length() - 1) + 
                          recursiveReverse(orig.substring(0, orig.length() - 1));
 
    }

}

Char Array (Best without built-ins)

package org.cloud.techtwitter.blogspot;
 
public class ReverseStringOptimal {

    public static void main(String[] args) {
        String s = "techTwitter";
        char[] arr = s.toCharArray();

        int left = 0, right = arr.length - 1;
        while (left < right) {
            char temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;
            left++;
            right--;
        }

        System.out.println(new String(arr));
    }
}

built-ins

String reversed = new StringBuilder("techTwitter").reverse().toString();

BEST Python Solution (No built-in reverse)

class ReverseString:

    def reverse(self, s: str) -> str:
        chars = list(s)              # convert string to list (mutable)
        left, right = 0, len(chars) - 1

        while left < right:
            chars[left], chars[right] = chars[right], chars[left]
            left += 1
            right -= 1

        return "".join(chars)


# ----- Caller / Main -----
if __name__ == "__main__":
    rs = ReverseString()
    input_str = "techTwitter"
    result = rs.reverse(input_str)
    print("Reversed string:", result)

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