Leaders in an Array
An easy Arrays problem included in Apna College, Love Babbar 450. Below: the roles whose interviews prioritise this topic, and how to practise it.
- Topic
- Arrays
- Sheets
- 2
- Core for
- 25 roles
- Platform
- GeeksforGeeks
The problem
Find all leaders in an array. An element is a leader if it is strictly greater than every element to its right.
Example 1
- Input
- [16,17,4,3,5,2]
- Output
- [17,5,2]
Example 2
- Input
- [1,2,3,4,5]
- Output
- [5]
Example 3
- Input
- [5,4,3,2,1]
- Output
- [5,4,3,2,1]
Constraints
- 1 <= n <= 10^5
- -10^9 <= arr[i] <= 10^9
How to think about it
Updated 2026-09-09Scanning left-to-right forces you to wonder what lies ahead for every element. Reversing direction and scanning right-to-left inverts that relationship: the running maximum of elements seen so far is all that decides whether the next element to the left is a leader.
Approaches, worst first
Forward quadratic inspection
time O(n^2) · space O(1)
For each index i from 0 to n - 1, scan all elements from i + 1 to n - 1 to verify if any element is greater than or equal to arr[i]. Retains original order naturally but repeats identical suffix scans for every single position.
Reverse suffix max scanWrite this one
time O(n) · space O(1)
Iterate backwards from index n - 1 down to 0, maintaining `maxFromRight`. If arr[i] is strictly greater than `maxFromRight`, collect arr[i] and update `maxFromRight`. Reversing the collected leaders at the end yields the expected left-to-right order.
Where people lose marks · 3
- The rightmost element (at index n - 1) has no elements to its right and is always a leader by definition; dropping it or failing to initialize correctly breaks.
- Equality condition: using `>=` instead of `>` violates the strict inequality rule when duplicate maximum values occur.
- Seeding `maxFromRight` with 0 instead of arr[n - 1] fails when all elements are negative.
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.
Companies that have asked it
Tags taken from the problem's own GeeksforGeeks page — not a copied list.
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