Find the Longest Consecutive Sequence
A medium Arrays problem included in Apna College, Love Babbar 450, Striver A2Z. Below: the roles whose interviews prioritise this topic, and how to practise it.
- Topic
- Arrays
- Sheets
- 3
- Core for
- 25 roles
- Platform
- LeetCode
The problem
Given an unsorted array, find the length of the longest sequence of consecutive integers.
Example 1
- Input
- [100,4,200,1,3,2]
- Output
- 4
Example 2
- Input
- [0,3,7,2,5,8,4,6,0,1]
- Output
- 9
Example 3
- Input
- [1]
- Output
- 1
Constraints
- 1 <= n <= 10^5
- -10^9 <= arr[i] <= 10^9
How to think about it
Updated 2026-09-09Every contiguous sequence has a unique first number x whose predecessor x - 1 does not exist anywhere in the array. Filtering for only those starting numbers before launching a sequence count guarantees that each number in the array is visited at most twice overall, yielding strict linear time without sorting.
Approaches, worst first
Sort and streak count
time O(n log n) · space O(1)
Sort the array ascending and scan through, incrementing a streak counter whenever arr[i] == arr[i - 1] + 1 and ignoring duplicate values. Correct and memory efficient, but bounded by the comparison sorting ceiling.
Hash set start filterWrite this one
time O(n) · space O(n)
Insert all numbers into a hash set. For each value x, check if x - 1 is present. If not, x is a streak initiator: increment a local count while x + 1, x + 2, etc. exist in the set. Maximize the streak length across all valid initiators.
Where people lose marks · 3
- Starting a while loop on every single element x instead of checking `!set.has(x - 1)` first degrades runtime to quadratic when given long consecutive runs like 1, 2, ..., n.
- Failing to ignore duplicate values in the sorting approach prematurely terminates streak counts.
- A single-element array where n = 1 must return 1 as the streak length.
The theory behind it
Arrays — the ground this problem stands on. All Arrays problems
What Arrays is
An array is a row of fixed boxes laid side by side in computer memory, like numbered lockers in a hallway. Because each box occupies identical space and sits directly next to its neighbors, jumping to locker zero or locker ten thousand takes the exact same tiny fraction of time. Every box holds an item of the same type, addressed by an offset number called an index.
When to reach for it
Reach for an array when items arrive in a known sequence and need immediate retrieval by position number. Problems asking for running totals, prefix accumulations, cyclic rotations, or in-place rearrangements signal array mechanics. Whenever constraints require constant-time random lookups or contiguous cache scans across fixed collections, a flat sequence is the default container.
How the pattern works
Visualize a tape with zero-indexed slots stretching from start to end. Keep track of write and read cursors when modifying contents without allocating helper buffers. For running computations, maintain an invariant such as having processed all elements left of the current index while pending elements wait to the right. When modifying entries in place, consider scanning backwards from the end so unread data is not overwritten.
What each operation costs
| Operation | Time |
|---|---|
| look up element by index | O(1) |
| insert or delete at the start | O(n) |
| search an unsorted collection for a value | O(n) |
What usually goes wrong with Arrays
- Reading past the final index by checking index less than or equal to length instead of strictly less than length, triggering index out of bounds exceptions.
- Modifying length or removing elements during a forward iteration loop, which causes remaining items to shift left and skip validation on the next neighbor.
- Assuming dynamic resizing is costless inside nested loops, causing repeated memory reallocation copies when appending unknown quantities of items.
Which roles need this problem
Arrays is a core topic for these 25 roles — if you're targeting one of them, this problem is early in your path, not optional.
Secondary for 3 more roles, including Database Engineer, Bioinformatics Engineer, Networking Engineer.
Track this in your role's order
Pick your target role and all 370 problems — including this one — resequence to what that interview actually asks. Free.
Start freeMore Arrays problems
Problem set and role mapping as of .