128 - Longest Consecutive Sequence

#medium

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 result = 0;
        unordered_set<int> set;
        for ( int i = 0; i < nums.size(); i++ ) {
            set.insert( nums[i] );
        }

        for ( int i = 0; i < nums.size(); i++ ) {
            if ( set.find( nums[i] ) != set.end() ) {
                int temp = 1, target = nums[i];
                set.erase( target );
                while ( set.find( --target ) != set.end() ) {
                    temp++;
                    set.erase( target );
                }

                target = nums[i];
                while ( set.find( ++target ) != set.end() ) {
                    temp++;
                    set.erase( target );
                }
                result = max( result, temp );
            }
        }
        
        return result;
    }
};