128. Longest Consecutive Sequence

Posted 2020-09-30

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

考虑了多种方法, 还是得遍历, 不过可以借助hashSet

public class Solution {
    public int longestConsecutive(int[] nums) {
          HashSet<Integer> set = new HashSet<Integer>();
        for (int i : nums) {
            set.add(i);
        }
        
        int ans = 0;
        for (int i : nums) {
           if (set.contains(i - 1)) {
               continue;
           }
           int temp = 0;
           while (set.contains(i++)) {
               temp++;
           }
           ans = Math.max(ans, temp);
       }

        return ans;
    }
}