Class 03 solutions

A.java

import java.util.Scanner;


public class A {
	
	public static String binaryString(int x, int n) {
		StringBuilder sb = new StringBuilder();
		
		sb.append(Integer.toString(x, 2));
		
		while (sb.length() < n) {
			sb.insert(0, '0');
		}
		
		return sb.toString();
	}
	
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		
		int N = in.nextInt();
		
		for (int count = 0; count < N; count++) {
			if (count != 0) {
				System.out.println();
			}
			
			int n = in.nextInt();
			int h = in.nextInt();
			
			for (int i = 0; i < (1 << n); i++) {
				if (Integer.bitCount(i) == h) {
					System.out.println(binaryString(i, n));
				}
			}
		}
	}

}

B.java

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


public class B {
	
	public static int twosComplement(int x) {
		return (x ^ 0xFFFFFFFF) + 1;
	}
	
	public static void printBinaryString(int x) {
		Stack<Character> stack = new Stack<Character>();
		
		while (x > 0) {
			char ch = (char) ('0' + (x & 1));
			stack.push(ch);
			x >>= 1;
		}
		
		while (!stack.isEmpty()) {
			System.out.print(stack.pop());
		}
		
		System.out.println();
	}
	
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		
		while (in.hasNextInt()) {
			int x = in.nextInt();
			
			// Really easy if you know the Java API
			// System.out.println(Integer.reverseBytes((int) x));
			
//			int y = (x & 0xFF000000) >> 24
//					| (x & 0x00FF0000) >> 8
//					| (x & 0x0000FF00) << 8
//					| (x & 0x000000FF) << 24;
			
			int y =   (x >> 24) & 0x000000FF
					| (x >>  8) & 0x0000FF00
					| (x <<  8) & 0x00FF0000
					| (x << 24) & 0xFF000000;

			System.out.printf("%d converts to %d\n", x, y);
		}
	}

}