Morgan And A String in Java

 Jack and Daniel are friends. Both of them like letters, especially uppercase ones.

They are cutting uppercase letters from newspapers, and each one of them has his collection of letters stored in a stack.

One beautiful day, Morgan visited Jack and Daniel. He saw their collections. He wondered what is the lexicographically minimal string made of those two collections. He can take a letter from a collection only when it is on the top of the stack. Morgan wants to use all of the letters in their collections.

As an example, assume Jack has collected  and Daniel has . The example shows the top at index  for each stack of letters. Assemble the string as follows:

Jack	Daniel	result
ACA	BCF
CA	BCF	A
CA	CF	AB
A	CF	ABC
A	CF	ABCA
    	F	ABCAC
    		ABCACF

Note the choice when there was a tie at CA and CF.

Function Description

Complete the morganAndString function in the editor below.

morganAndString has the following parameter(s):

• string a: Jack's letters, top at index 
• string b: Daniel's letters, top at index 

Returns
- string: the completed string

Input Format

The first line contains the an integer , the number of test cases.

The next  pairs of lines are as follows:
- The first line contains string 
- The second line contains string .

Constraints

•  and  contain upper-case letters only, ascii[A-Z].

Sample Input

2
JACK
DANIEL
ABACABA
ABACABA

Sample Output

DAJACKNIEL
AABABACABACABA

Explanation

The first letters to choose from are J and D since they are at the top of the stack. D is chosen and the options now are J and A. A is chosen. Then the two stacks have J and N, so J is chosen. The current string is DA. Continue this way to the end to get the string.

SOLUTION:

import java.io.*;

import java.util.*;

import java.text.*;

import java.math.*;

import java.util.regex.*;

public class Solution {

    private static final int MAXSIZE = 200100;

    private static final int ALPHABET = 128;

    

    public static void main(String[] args) {

      

        Scanner in = new Scanner(System.in);

        int testcase = Integer.parseInt(in.nextLine());

        for (int i = 0; i < testcase; i++) {

            String str1 = in.nextLine();

            String str2 = in.nextLine();

            System.out.println(solution(str1 + "a", str2 + "b"));

        }

    }

    private static List<Integer> buildSuffixArray(String str) {

        int[] p = new int[MAXSIZE];

        int[] c = new int[MAXSIZE];

        int[] cnt = new int[MAXSIZE];

        int[] pn = new int[MAXSIZE];

        int[] cn = new int[MAXSIZE];

        Arrays.fill(cnt, 0);

        int n = str.length();

        for (int i = 0; i < n; i++) {

            ++cnt[str.charAt(i)];

        }

        for (int i = 1; i < ALPHABET; i++) {

            cnt[i] += cnt[i - 1];

        }

        for (int i = 0; i < n; i++) {

            p[--cnt[str.charAt(i)]] = i;

        }

        int count = 1;

        c[p[0]] = count - 1;

        for (int i = 1; i < n; i++) {

            if (str.charAt(p[i]) != str.charAt(p[i - 1])) {

                ++count;

            }

            c[p[i]] = count - 1;

        }

        for (int h = 0; (1 << h) < n; ++h) {

            for (int i =0; i < n; i++) {

                pn[i] = p[i] - (1 << h);

                if (pn[i] < 0) {

                    pn[i] += n;

                }

            }

            Arrays.fill(cnt, 0);

            for (int i = 0; i < n; i++) {

                ++cnt[c[i]];

            }

            for (int i = 1; i < count; i++) {

                cnt[i] += cnt[i - 1];

            }

            for (int i = n - 1; i >= 0; i--) {

                p[--cnt[c[pn[i]]]] = pn[i];

            }

            count = 1;

            cn[p[0]] = count - 1;

            for (int i = 1; i < n; i++) {

                int pos1 = (p[i] + (1 << h)) % n;

                int pos2 = (p[i - 1] +  (1 << h)) % n;

                if (c[p[i]] != c[p[i - 1]] || c[pos1] != c[pos2]) {

                    ++count;

                }

                cn[p[i]] = count - 1;

            }

            for (int i = 0; i < n; i++) {

                c[i] = cn[i];

            }

        }

        List<Integer> res = new ArrayList<Integer>(n);

        for (int i = 0; i < n; i++) {

            res.add(c[i]);

        }

        return res;

    }

    

    private static String solution(String str1, String str2) {

        StringBuilder sb = new StringBuilder(str1).append(str2);

        List<Integer> suffix = buildSuffixArray(sb.toString());

        StringBuilder rst = new StringBuilder();

        int start1 = 0;

        int start2 = 0;

        while (start1 < str1.length() - 1 || start2 < str2.length() - 1) {

            if (start1 >= str1.length() - 1) {

                rst.append(str2.charAt(start2++));

                continue;

            }

            if (start2 >= str2.length() - 1) {

                rst.append(str1.charAt(start1++));

                continue;

            }

            if (suffix.get(start1) < suffix.get(str1.length() + start2)) {

                rst.append(str1.charAt(start1++));

            } else {

                rst.append(str2.charAt(start2++));

            }

        }

        return rst.toString();

    }

}