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# Longest Consecutive Sequence

Moderate
0/80
Average time to solve is 40m
Contributed by
168 upvotes
Asked in companies

## Problem statement

You are given an unsorted array/list 'ARR' of 'N' integers. Your task is to return the length of the longest consecutive sequence.

The consecutive sequence is in the form ['NUM', 'NUM' + 1, 'NUM' + 2, ..., 'NUM' + L] where 'NUM' is the starting integer of the sequence and 'L' + 1 is the length of the sequence.

Note:

``````If there are any duplicates in the given array we will count only one of them in the consecutive sequence.
``````
For example-
``````For the given 'ARR' [9,5,4,9,10,10,6].

Output = 3
The longest consecutive sequence is [4,5,6].
``````
Follow Up:
``````Can you solve this in O(N) time and O(N) space complexity?
``````
Detailed explanation ( Input/output format, Notes, Images )
Constraints :
``````1 <= T <= 10
1 <= N <= 10^5
-10^9 <= ARR[i] <= 10^9

Time Limit: 1 sec
``````
Sample Input 1 :
``````1
5
33 20 34 30 35
``````
Sample Output 1 :
``````3
``````
Explanation to Sample Input 1 :
``````The longest consecutive sequence is [33, 34, 35].
``````
Sample Input 2 :
``````1
7
1 9 3 10 4 20 2
``````
Sample Output 2 :
``````4
``````
Explanation to Sample Input 2 :
``````The consecutive sequence is in the form ['NUM', 'NUM' + 1, 'NUM' + 2,...,'NUM' + 'L']. So in the given array, the longest consecutive sequence is [1,2,3,4] where 'NUM' = 1 and 'L' = 3. And the length of the sequence will be 'L' + 1 = 4.
``````
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