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.

Naive Sorting Solution

• sort the sequence and find the consecutive subsequences and out of them return the length of the longest consecutive subsequence.
• TC: O(NlogN)

Hash table O(N) Time solution

• we create hash table of nums.
• for every element we check if num-1 is present or not.
• if its not present then from num we count num,num+1,num+2,... consecutive elements.
• when num+.. not found we update our ans.
• return ans.

Code

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        int n = nums.size(), cnt=0, ans=0;
        if(n==0)return 0;
        unordered_set<int> st;
        for(auto x:nums)st.insert(x);
        for(auto x:nums){
            if(st.count(x-1)!=1){
                int currentNum = x;
                cnt=1;
                while(st.count(currentNum+1)){
                    currentNum++;
                    cnt++;
                }
                ans=max(ans,cnt);
            }
        }
        return ans;
    }
};