MorganString.java

import java.io.IOException;
import java.util.Stack;

public class Solution {

    // Complete the morganAndString function below.
    static String morganAndString(String a, String b) {
        StringBuilder builder = new StringBuilder();
        Stack<String> stackA = new Stack();
        Stack<String> stackB = new Stack();

        char[] charsA = a.toCharArray();
        char[] charsB = b.toCharArray();

        for (int i = charsA.length - 1; i >= 0; i--) {
            stackA.push(Character.toString(charsA[i]));
        }

        for (int i = charsB.length - 1; i >= 0; i--) {
            stackB.push(Character.toString(charsB[i]));
        }

        while (!stackA.empty() && !stackB.empty()) {
            builder.append(compareAndPopSmallLetter(stackA, stackB));
        }

        if (stackA.empty()) {
            while (!stackB.empty()) {
                builder.append(compareAndPopSmallLetter(stackB, stackB));
            }
        } else if (stackB.empty()) {
            while (!stackA.empty()) {
                builder.append(compareAndPopSmallLetter(stackA, stackA));
            }
        }

        return builder.toString();
    }

    private static String compareAndPopSmallLetter(Stack<String> stackA, Stack<String> stackB) {
        return stackA.peek().charAt(0) < stackB.peek().charAt(0) ? stackA.pop() : stackB.pop();
    }

    public static void main(String[] args) throws IOException {
        System.out.println(morganAndString("DADBC", "DADB"));
    }
}