DSA interview question

Leaders in an Array

Easy · Algorithms · Topics: array

The problem

A marathon's live tracker lists the runners' current speeds from the front of the pack to the back, and marks a runner as a leader if nobody behind them is faster, meaning their speed is greater than or equal to every speed to their right. The last runner is always a leader. Checking every runner against everyone behind them is O(n²). Scanning from the right instead, keeping the maximum seen so far, identifies all leaders in one pass.

Example: [16, 17, 4, 3, 5, 2] → [17, 5, 2].

Function signature

vector<int> leaders(const vector<int>& a);  // leaders in their original left-to-right order

Test cases

Input `a`	Output	Explanation
`[16, 17, 4, 3, 5, 2]`	`[17, 5, 2]`
`[10, 4, 2, 4, 1]`	`[10, 4, 4, 1]`	Equal to the maximum on the right still counts
`[5, 4, 3, 2, 1]`	`[5, 4, 3, 2, 1]`	Every element is a leader
`[1, 2, 3, 4, 5]`	`[5]`	Only the last element
`[7]`	`[7]`
`[1, 1, 1]`	`[1, 1, 1]`

(Confirm: a leader is greater than or equal to every element to its right (the GeeksforGeeks definition); output in original order.)

How interviewers phrase it

"Find all leaders in an array" (GeeksforGeeks).
Related: "Replace every element with the greatest element on its right" (LeetCode 1299), or "next greater element" (a stack problem).

Clarifying questions to ask

Greater than, or greater than or equal to, the elements on the right?
Should the output be in original order?
Can values be negative?

Follow-ups to expect

"Why scan from the right?"
"How do you output leaders in the original order?"
"What changes for a strict definition (greater than)?"
"What's the brute force?"
"Complexity?"

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