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Longest Consecutive Sequence

Tags : leetcode, cpp, medium

Problem Statement - link #

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.

Examples #

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 #

Solutions #

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
      set<int> s;
      for(auto x: nums) s.insert(x);
      int res = 0;
      for(auto i: s)
        if(s.find(i-1)==s.end()){
          int x = i;
          int temp = 1;
          while(s.find(x+1)!=s.end()){
            x+=1; temp+=1;
          }
          res = max(res, temp);
        }
      
      return res;
    }
};