Solution.java

// github.com/RodneyShag

import java.util.Scanner;
import java.util.Stack;

//  Time Complexity: O(n)
// Space Complexity: O(n)

// Can alternatively use an ArrayDeque instead of a Stack. Just make sure to iterate
// through it properly since iteration order is opposite that of a Stack.

String superReducedString(String str) {
    /* Iterate through String, creating final result in a Stack */
    Stack<Character> stack = new Stack();
    for (int i = 0; i < str.length(); i++) {
        Character ch = str.charAt(i);
        if (!stack.isEmpty() && ch == stack.peek()) {
            stack.pop(); // since String has 2 adjacent equal characters
        } else {
            stack.push(ch);
        }
    }
    
    /* Return final result */
    if (stack.isEmpty()) {
        return "Empty String";
    } else {
        StringBuffer sb = new StringBuffer();
        for (char ch : stack) {
            sb.append(ch);
        }
        return sb.toString();
    }
}

// Discuss on HackerRank: https://www.hackerrank.com/challenges/reduced-string/forum/comments/263979