Longest Consecutive Sequence

Posted 2023-01-20 IIcyZhao

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.

class Solution 
public:
    int longestConsecutive(vector<int> &num) 
       unordered_map<int, bool> map;
       for (int i = 0; i < num.size(); ++i) 
           map.insert(num[i], false);
       
       int res = 0;
       for (int i = 0; i < num.size(); ++i) 
           if (map[num[i]]) 
               continue;
           
           int low = high = num[i];
           while (0 != map.count(high)) 
               map[high++] = true;
           
           while (0 != map.count(low)) 
               map[low--] = true;
           
           int cnt = high - low + 1;
           res = max(res, cnt);
       
       return res; 
    
;