Quiz - 2020

package com.bogotobogo;
public class Reverser {
 		public String reverse(String input) {
 			/* Method A
 			 * return new StringBuilder(input).reverse().toString(); 
 			 */
 			
 			/* Method B */
 			char[] a = input.toCharArray(); 
 			String rev = "";
 			for(int i = a.length-1; i >= 0;i--)  rev += a[i];
 			return rev;
 		}
 		public static void main (String[] args) {
 			System.out.println("in="+args[0]);
 			System.out.println("out="+new Reverser().reverse(args[0]));
 		}
}

Output:

in=123456
out=654321

Here is an array example: how to create and to pass into a method:

import java.util.*;

public class test {
	public static void main(String[] args) {
		System.out.println("Binary search");
		(new test()).go();
	}
	
	public void go() {
		int[] array = new int[]{0,1,2,3,4,5,6,7,8,9};
		int x = BinarySearch(array, 4);
	        if(x == -1)	
	    	    System.out.println(x+" is not in the array!");
	        else
	    	    System.out.println(x+" is at " + x);				
	}
	
	public int BinarySearch(int[] a, int n) {
		int upper = a.length - 1;
		int lower = 0;
		int mid = 0;
		
		while(lower <= upper) {
			mid = (upper + lower)/2;
			if(n < a[mid])
				upper = mid - 1;
			else if(n > a[mid])
				lower = mid + 1;
			else 
				return mid;
		}
		return -1;
	}
}

Output:

Binary search
4 is at 4

The following code is a slightly modified version using ArrayList instead of the simple integer array. I made some changes to the code: two classes instead of just one.

// test.java

package com.bogo;

import java.util.ArrayList;

public class test {
	public static void main(String[] args) {
		System.out.println("Binary search");
		(new test()).go();
	}
	
	public void go() {
		ArrayList<Integer> array = new ArrayList<Integer>();
		for(int i = 0; i < 10; i++) {
			array.add(i);
		}
		Algorithms al = new Algorithms();
		int x = al.BinarySearchArrayList(array, 4);
	        if(x == -1)	
	    	    System.out.println(x+" is not in the array!");
	        else
	    	    System.out.println(x+" is at " + x);		
		
	}
}

// Algorithms.java

package com.bogo;

import java.util.ArrayList;

public class Algorithms {
	
	public int BinarySearch(int[] a, int n) {
		int upper = a.length - 1;
		int lower = 0;
		int mid = 0;
		
		while(lower <= upper) {
			mid = (upper + lower)/2;
			if(n < a[mid])
				upper = mid - 1;
			else if(n > a[mid])
				lower = mid + 1;
			else 
				return mid;
		}
		return -1;
	}
	
	public int BinarySearchArrayList(ArrayList<Integer> a, int n) {
		int upper = a.size() - 1;
		int lower = 0;
		int mid = 0;
		
		while(lower <= upper) {
			mid = (upper + lower)/2;
			if(n < a.get(mid))
				upper = mid - 1;
			else if(n > a.get(mid))
				lower = mid + 1;
			else 
				return mid;
		}
		return -1;
	}
}

//test.java

package com.bogotobogo;

public class test {
	public static void main(String[] args) {
		System.out.println("String Permutation");
		(new test()).go();
	}
	
	public void go() {
		String s = "1234";	
		Permutations(s);
	}
	
	public void Permutations(String s) {
		int length = s.length();
		boolean[] used = new boolean[length];
		char[] input = s.toCharArray();
		StringBuffer output = new StringBuffer();
		int level = 0;
		DoPermutations(input, output, used, length, level);
	}
	
	public void DoPermutations(char[] input, StringBuffer output, boolean[] used, int length, int level) {
		if(length == level) {
			System.out.println(output);
			return;
		}
		for(int i = 0; i < length; i++) {
			if(used[i] == true) continue;
			output.append(input[i]);
 			used[i] = true;
			DoPermutations(input, output, used, length, level+1);
			used[i] = false;
			output.setLength(output.length()-1);
		}
	}
}

Output is:

String Permutation
1234
1243
1324
1342
1423
1432
2134
2143
2314
2341
2413
2431
3124
3142
3214
3241
3412
3421
4123
4132
4213
4231
4312
4321