Question

I’ve got a collection of strings, and I need to know the first index where they all differ. I can think of two ways to do this: (the following pseudo code is just off the top of my head and may be heavily bug-laden)

First Way:

var minLength = [go through all strings finding min length];
var set = new set()
for(i=0;i<minlength;i++)
{
  for(str in strings)
  {
    var substring = str.substring(0,i);
    if(set.contains(substring))
      break; // not all different yet, increment i
    set.add(substring)
  }
  set.clear(); // prepare for next length of substring
}

This strikes me as gross because of the use of a set data structure where it seems like one should not be needed.

Second Way:

var minLength = [go through all strings finding min length];
strings.sort();
for(i=0;i<minlength;i++)
{
  boolean done = true;
  char last = null;
  for(str in strings)
  {
    char c = str[i];
    if(c == last)
    {
      // not all different yet, increment i
      done = false;
      break;
    }
    last = c;
  }
  if(done)
    return i;
}

But it annoys me that I have to run the sort first, because the sorting algorithm, by its very nature, has access to the information that I’m looking for.

Surely there must be a more efficient way than what I have listed above. Eventually I’d like to abstract it out to any type of array, but that will be trivial and it’s simpler to think of it as a string problem.

Any help?

**UPDATE: I apparently didn’t explain myself very well. If my strings are [“apple”, “banana”, “cucumber”, “banking”], I want the function to return 3, because there were two strings (“banana” and “banking”) that matched through index 0, 1, and 2, so 3 is the first index where they are all unique.

As Daniel mentioned below, a better way to state my needs is that: “I want to find index i where calling substring(0,i) on all my strings will result in all unique values.”**

Answer 1

Use the set as you proposed, that’s exactly the right thing to do.