Consider the following function:

int f (int num)
{
int out = 0;

for (; num 0; num /= 10) {
int d = num % 10;
out *= 10;
out += d;
}

return out;
}

1) What does it do?

2) Write the same algorithm using recursion

Question

Consider the following function:

	int f (int num)
        {
                int out = 0;

                for (; num > 0; num /= 10) {
                        int d = num % 10;
                        out *= 10;
                        out += d;
                }

                return out;
        }

1) What does it do?

2) Write the same algorithm using recursion

yf · Accepted Answer

Sorry my answer previously posted was wrong. Here is the correct one without golbal or static variables, which is basically the same as the first one by Paul.

inf f(int num, int car=0)
{
  car *=10;
  if((num/10) == 0) 
    return ( car + num );
 else
    return f(num/10, car+(num%10) ); 
}

Greatbear · Answer

private static int reverse(int n, int r) {
  if (n == 0) return r;
  return reverse(n/10, 10*r+n%10);
}
public static int reverse(int n) {
 return reverse(n, 0);
}

Deven · Answer

Simple method without using global variable.

static int reverseRecursive(int number, int sum) {
		if (number != 0) {
			int temp = number % 10;
			sum = sum * 10 + temp;
			return reverseRecursive((number / 10), sum);
		} else
			return sum;

	}

Paul · Answer

There is a trick that need global variable or static variable to record the temper sum. so the function declaration cannot be same as loop version if don't use global or static variable. 

the output parameter b will be the reverse result.

void rev(int d, int& b)
{
	b *= 10;
	if (d/10 == 0)
		b += d;
	else
	{
		b += d%10;
		rev(d/10, b);
	}
}

yf · Answer

You can do it without the golbal or static variables:

int f(int num)
{
if((num/10) == 0) 
  return num;
else
 return ( 10(num%10) + f(num/10) );
}

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