1. class Solution {
  2.     /**
  3.     1. Store all elements in a HashSet, This allows O(1) lookup for checking if a number exists.
  4.     2. Only start building a sequence from numbers that are the beginning 
  5.         - number is the start if, set does not contains (element-1)
  6.         - count (currentLength) how long the sequence goes, so Expand forward (ele + 1, ele + 2, …)
  7.         - once you found the current length of sequence, Track the maximum length
  8.     3. repeat step-2 for all the array elements
  9.     4. return the max length
  10.     */
  11.     public int longestConsecutive(int[] nums) {
  12.         if(nums.length == 0)
  13.             return 0;
  14.  
  15.         // to remove duplicates
  16.         HashSet<Integer> set = new HashSet<>();
  17.         for(int ele : nums)
  18.             set.add(ele);
  19.  
  20.         int maxLength = 0;
  21.         for(int ele : nums)
  22.         {
  23.             // find the start of sequence
  24.             if(set.contains(ele - 1) == false) // this is not the start
  25.             {
  26.                 int length = 1;
  27.                 while(set.contains(ele + 1))
  28.                 {
  29.                     ele++;
  30.                     length++;
  31.                 }
  32.                 maxLength = Math.max(length, maxLength);
  33.             }
  34.         }
  35.         return maxLength;
  36.     }
  37. }