Longest Consequent Sequence@leetcode

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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.

» Solve this problem

Because it has to be solved in O(n) time. Thus, normal sorting algorithm cannot be applied here. So hashmap came to my mind, to quick allocate the element.

Algorithm:
1. Create a hashmap to store the given sequence
2. Create a vector to mark visited elements and avoid duplicate search
3. search num[i] +1 and num[i]-1 iteratively, and set count.
4. Return the longest length we found.