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.

Sorting Solution

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

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;
    }
};