06. Longest Consecutive Sequence.cpp

/*
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.

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.

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

Constraints:
0 <= nums.length <= 105
-109 <= nums[i] <= 109
*/

class Solution
{
public:
  int longestConsecutive(vector<int> &nums)
  {
    int n = nums.size(), ans = 0;
    unordered_set<int> vis;
    for (auto &i : nums)
      vis.insert(i);

    for (auto i : nums)
    {
      if (vis.count(i - 1))
        continue;
      else
      {
        int count = 0;
        while (vis.count(i))
        {
          count++;
          i++;
        }
        ans = max(ans, count);
      }
    }

    return ans;
  }
};