10진수를 2 진수로 변환

my attempt

public static void main(String[] args) {
	System.out.println(solution(10));
	System.out.println(solution(27));
	System.out.println(solution(12345)); // error here
}
    
public static int solution(int num) {
	Stack<Integer> stack = new Stack<>();
	while (num > 0) {
		int r = num % 2;
		stack.push(r);
		num /= 2;
	}
	String res = "";
	while (!stack.isEmpty()) {
		res += stack.pop();
	}
	return Integer.parseInt(res);
}

• critique
  • I should use a StringBuilder instead of String because adding something to String
  • When concatenating to String using + it creates a new object each time → so use StringBuilder
• A problem:
  • if i input 12345, java throws a Exception in thread "main" java.lang.NumberFormatException: For input string: "11000000111001"
  • The string generated ("11000000111001") represents a number that’s too large to fit into Java int
    • it’s about 11 trillion!
  • A maximum value a 32-bit int can hold is ~2 billion
  • the standard approach is to return a String

solution

public static void main(String[] args) {  
    System.out.println(solution(10));  
    System.out.println(solution(27));  
    System.out.println(solution(12345));  
}  
  
public static String solution(int num) {  
    Stack<Integer> stack = new Stack<>();  
    while (num > 0) {  
        int r = num % 2;  
        stack.push(r);  
        num /= 2;  
    }  
    StringBuilder sb = new StringBuilder();  
    while (!stack.isEmpty()) {  
        sb.append(stack.pop());  
    }  
    return sb.toString();  
}

time complexity

• O(log N)
  • Since you keep dividing by 2 it’s log N