#128 Longest consecutive sequence

Posted 2021-01-18 yxcindy

两种方法：

1. 先sort，再找。time complexity: O（nlogn）如果用array记录次数，space complexity是O（n）。如果只用int来记录current length以及longest length, space complexity 是O(1)

2. 用hashset。Time complexity O(n), space complexity O(n).

   public int longestConsecutive(int[] nums) {

       Set<Integer> num_set = new HashSet<Integer>();

       for (int num : nums) {

           num_set.add(num);

       }

 

       int longestStreak = 0;

       for (int num : num_set) {

           if (!num_set.contains(num-1)) {

               int currentNum = num;

               int currentStreak = 1;

               while (num_set.contains(currentNum+1)) {

                   currentNum += 1;

                   currentStreak += 1;

               }

               longestStreak = Math.max(longestStreak, currentStreak);

           }

       }

       return longestStreak;

   }