# Longest Consecutive Sequence Problem

## Description

LeetCode Problem 128.

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:

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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:

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Input: nums = [0,3,7,2,5,8,4,6,0,1]
Output: 9
``````

Constraints:

• 0 <= nums.length <= 10^5
• -10^9 <= nums[i] <= 10^9

## Sample C++ Code

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int longestConsecutive(vector<int>& nums) {
// inserting all elements into hashset
unordered_set<int> s(begin(nums), end(nums));
int longest = 0;
for(auto& num : s) {
int cur_longest = 1;
// find consecutive elements in the backward and forward direction from num
for(int j = 1; s.count(num - j); j++) {
s.erase(num - j);
cur_longest++;
}
for(int j = 1; s.count(num + j); j++) {
s.erase(num + j);
cur_longest++;
}
// update longest to hold longest consecutive sequence till now
longest = max(longest, cur_longest);
}
return longest;
}
``````