Find The Longest Consecutive Sequence In An Array In Ruby
Identify the longest run of consecutive integers in an unsorted array — useful for streak detection, range analysis, and data validation.
Description
Sort the array, remove duplicates, then use chunk_while to group consecutive integers (where each next element equals the previous plus one). The longest group is found with max_by(&:length).
Alternatively, use a set for O(n) lookup: for each element that starts a sequence (has no predecessor in the set), walk forward until the chain breaks.
Sample input:
nums = [100, 4, 200, 1, 3, 2, 5, 6]
Sample Output:
[1, 2, 3, 4, 5, 6] # length 6
Answer
nums = [100, 4, 200, 1, 3, 2, 5, 6]
# chunk_while approach — clean and idiomatic
longest = nums.sort.uniq
.chunk_while { |a, b| b == a + 1 }
.max_by(&:length)
# => [1, 2, 3, 4, 5, 6]
longest.length # => 6
longest.first # => 1 (start of sequence)
longest.last # => 6 (end of sequence)
# O(n) set approach — faster for large inputs
require 'set'
def longest_consecutive(nums)
set = nums.to_set
longest = []
set.each do |n|
next if set.include?(n - 1) # only start from sequence beginnings
seq = n.step.lazy.take_while { |x| set.include?(x) }.to_a
longest = seq if seq.length > longest.length
end
longest
end
longest_consecutive(nums)
# => [1, 2, 3, 4, 5, 6]
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