DSA interview question

Second Largest Element in an Array

Easy · Algorithms · Topics: array, linear search

The problem

A leaderboard shows the top score and, right below it, the runner-up score, which must be strictly less than the top score (if two players tie for first, the runner-up is the next distinct score). If every player has the same score, there's no runner-up, and the screen shows "-1". Sorting all the scores just to read two values is wasteful; the classic interview answer finds both in a single pass by tracking the largest and second-largest so far.

Example: [12, 35, 1, 10, 34, 1] → 34.

Function signature

int secondLargest(const vector<int>& a);  // largest value strictly below the maximum, or -1 if none

Test cases

Input `a`	Output	Explanation
`[12, 35, 1, 10, 34, 1]`	34
`[10, 5, 10]`	5	The duplicate maximum doesn't count as second
`[10, 10, 10]`	-1	No value below the maximum
`[7]`	-1	Only one element
`[1, 2]`	1
`[2, 1]`	1	Order doesn't matter

(Confirm: values are positive (as in the GeeksforGeeks version), so -1 unambiguously means "no second largest".)

How interviewers phrase it

"Find the second largest element in an array" (GeeksforGeeks).
"Find the second largest and second smallest elements without sorting."
As a lead-in to "find the k-th largest element".

Clarifying questions to ask

Must the second largest be strictly smaller than the largest, or can duplicates of the max count?
What should be returned if it doesn't exist?
Can values be negative? (Then -1 is a poor sentinel.)
Is one pass required?

Follow-ups to expect

"What are the update rules?"
"Why not sort?"
"What if values can be negative?"
"How would you get the k-th largest?"
"Second smallest?"

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