Longest Consecutive Sequence in an Unsorted Array [duplicate]

Answer 1

Dump everything to a hash set.

Now go through the hashset. For each element, look up the set for all values neighboring the current value. Keep track of the largest sequence you can find, while removing the elements found from the set. Save the count for comparison.

Repeat this until the hashset is empty.

Assuming lookup, insertion and deletion are O(1) time, this algorithm would be O(N) time.

Pseudo code:

 int start, end, max
 int temp_start, temp_end, count

 hashset numbers

 for element in array:
     numbers.add(element)

 while !numbers.empty():
     number = numbers[0]
     count = 1
     temp_start, temp_end = number 

     while numbers.contains(number - 1):
         temp_start = number - 1; count++
         numbers.remove(number - 1)

     while numbers.contains(number + 1):
         temp_end = number + 1; count++
         numbers.remove(number + 1)

     if max < count:
         max = count
         start = temp_start; end = temp_end

 max_range = range(start, end)

The nested whiles don't look pretty, but each number should be used only once so should be O(N).