DSA interview question

Stock Span

Hard · Data structures · Topics: stack, monotonic stack

The problem

A brokerage app shows, next to each day's closing price, its span: the number of consecutive days up to and including today on which the price was ≤ today's price. A span of 6 means the stock is at a six-day high. Walking backwards from each day is O(n²) on long rising runs. A stack of previous higher prices fixes that. Pop every earlier day whose price is ≤ today's (they're absorbed into today's span), and the remaining top is the previous strictly higher day. The span is the distance to it.

Example: [100, 80, 60, 70, 60, 75, 85] → [1, 1, 1, 2, 1, 4, 6].

Function signature

vector<int> calculateSpan(const vector<int>& price);

Test cases

Input	Output	Explanation
`[100, 80, 60, 70, 60, 75, 85]`	`[1, 1, 1, 2, 1, 4, 6]`	85 covers everything back to after 100
`[10, 4, 5, 90, 120, 80]`	`[1, 1, 2, 4, 5, 1]`
`[5, 5, 5]`	`[1, 2, 3]`	Equal prices count (≤)
`[3, 2, 1]`	`[1, 1, 1]`
`[1, 2, 3]`	`[1, 2, 3]`

(Confirm: the span includes today; it counts consecutive previous days with price ≤ today's.)

How interviewers phrase it

"Calculate the span of the stock's price for all n days" (GeeksforGeeks "Stock span problem").
The online version is LeetCode 901 "Online Stock Span": a class whose next(price) returns today's span.

Clarifying questions to ask

Do equal prices count? (Yes: ≤.)
Does the span include today? (Yes.)
Offline array or online stream?

Follow-ups to expect

"What does the span relate to?"
"Why pop days with price ≤ today?"
"Online version?"
"Why O(n)?"
"Which classic problem is this?"

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