DSA interview question

Lemonade Change

Easy · Algorithms · Topics: greedy, array, simulation

The problem

A lemonade stand sells each cup for $5. Customers arrive in order and pay with a $5, $10 or $20 bill, and the stand starts with no change. Can it give every customer correct change? Only $5 and $10 bills are ever useful as change. A $10 needs one $5 back. A $20 needs $15 back: either $10 + $5 or $5 + $5 + $5. Greedy prefers $10 + $5, because $5 bills are more versatile (they're the only way to change a $10).

Example: [5, 5, 5, 10, 20] → true.

Function signature

bool lemonadeChange(const vector<int>& bills);

Test cases

Input	Output	Explanation
`[5, 5, 5, 10, 20]`	`true`	The $20 gets $10 + $5
`[5, 5, 10, 10, 20]`	`false`	Only $10 bills left, no $5
`[10, 10]`	`false`	No change for the first customer
`[5, 10, 5, 20]`	`true`
`[5, 5, 5, 5, 20, 20]`	`false`	The first $20 uses three $5s, then there's too little left

(Confirm: each cup costs exactly $5; the stand starts with no money; bills are only 5, 10 or 20.)

How interviewers phrase it

"Each lemonade costs $5. Customers pay with $5, $10 or $20 bills. Return true if you can provide every customer with correct change" (LeetCode 860).

Clarifying questions to ask

Does the stand start with no change? (Yes.)
Are the bills only 5, 10 and 20? (Yes.)
Must customers be served in the given order? (Yes.)

Follow-ups to expect

"Why don't you need to track $20 bills?"
"Why prefer $10 + $5 for a $20?"
"Prove greedy never fails when a solution exists."
"What if prices or denominations varied?"
"Complexity?"

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