Question

Is it possible to find the 2 largest numbers in an array, and only loop through the collection once?

I had this as an interview question and didn’t really get it in time.

Answer 1

It seems pretty straightforward..

int[] nums = { 3, 1, 4, 1, 5, 9, 2, 6 };

int max1 = -1;
int max2 = -1;
foreach (int num in nums)
{
  if (num > max1) { max2 = max1; max1 = num; }
  else if (num > max2) { max2 = num; }
}    

For example:

// 3: max2 = -1; max1 = 3;
// 1: max2 = 1;
// 4: max2 = 3; max1 = 4;

Quick explanation:

• Define -1 as a placeholder, could use int.MinValue, or better yet a separate bool to indicate no-match
• If the value tested is bigger than the current maximum (max1), assign current maximum to max2, new value to max1
• Else if must be smaller, but if it’s bigger than second-maximum, assign new value to max2