#275 Price Stack With Instant Minimum

A pricing editor lets you try price changes and undo them, last one first. It also shows the lowest price among all changes still applied. Write a class PriceStack:

• push(price) applies a change.
• pop() undoes the most recent change and returns nothing.
• top() returns the most recent price.
• getMin() returns the lowest price currently in the stack.

Every method must run in O(1) time. pop, top, and getMin are only called when the stack is not empty.

Each example lists the calls in order, then each call's arguments. The first call creates the object with new PriceStack(…); the output lists what each call returns (null for the constructor and for methods that return nothing).

Input: ["PriceStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]
Output: [null,null,null,null,-3,null,0,-2]

Input: ["PriceStack","push","top","getMin"]
[[],[7],[],[]]
Output: [null,null,7,7]

Constraints

• -2^31 <= price <= 2^31 - 1
• At most 3 * 10^4 calls.