128. Longest Consecutive Sequence

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

You must write an algorithm that runs in O(n) time.

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Example 1:

Input: nums = [100,4,200,1,3,2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

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Example 2:

Input: nums = [0,3,7,2,5,8,4,6,0,1]
Output: 9

Constraints:

  • 0 <= nums.length <= 105
  • -10^9 <= nums[i] <= 10^9

DFS Solution

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class Solution {
    public int longestConsecutive(int[] nums) {
        int n = nums.length;
        if (n == 0) return 0;
        Arrays.sort(nums);
        int max = 1;
        int currentMax = 1;
        for (int i = 1; i < n ; i++) {
            if (nums[i] - nums[i - 1] == 0) {
                continue;
            } else if (nums[i] - nums[i - 1] == 1) {
                currentMax++;
                max = Math.max(max, currentMax);
            } else {
                currentMax = 1;
            }   
        }
        return max;
    }
}