128. Longest Consecutive Sequence

[stada526]

Problem Link

https://leetcode.com/problems/longest-consecutive-sequence/description/

Solution

A solution that springs to mind is to sort the input nums in ascending order and traverse the sorted array.
During the traversal, if the adjacent elements are consecutive, increment a counter.
If the current element is not consecutive to the previous element, reset the counter to 0 and start again.
This approach operations in O(n * log n ) time complexity, but we're supposed to solve this problem in O(n).

For a more efficient approach, consider traversing the unsorted nums directly.
In each iteration, if the current element is the start of a sequence, determine how many consecutive elements are contained in nums staring from that element.
An element is considered the start of a sequence if the value element -1 is not present in nume.
To do this with a time complexity of O(n), use a hash set to store all elements of nums