0716. Max Stack

https://leetcode.com/problems/max-stack

Description

Design a max stack data structure that supports the stack operations and supports finding the stack's maximum element.

Implement the MaxStack class:

• MaxStack() Initializes the stack object.

• void push(int x) Pushes element x onto the stack.

• int pop() Removes the element on top of the stack and returns it.

• int top() Gets the element on the top of the stack without removing it.

• int peekMax() Retrieves the maximum element in the stack without removing it.

• int popMax() Retrieves the maximum element in the stack and removes it. If there is more than one maximum element, only remove the top-most one.

Example 1:

**Input**
["MaxStack", "push", "push", "push", "top", "popMax", "top", "peekMax", "pop", "top"]
[[], [5], [1], [5], [], [], [], [], [], []]
**Output**
[null, null, null, null, 5, 5, 1, 5, 1, 5]
**Explanation**
MaxStack stk = new MaxStack();
stk.push(5);   // [**5**] the top of the stack and the maximum number is 5.
stk.push(1);   // [5, **1**] the top of the stack is 1, but the maximum is 5.
stk.push(5);   // [5, 1, **5**] the top of the stack is 5, which is also the maximum, because it is the top most one.
stk.top();     // return 5, [5, 1, **5**] the stack did not change.
stk.popMax();  // return 5, [5, **1**] the stack is changed now, and the top is different from the max.
stk.top();     // return 1, [5, **1**] the stack did not change.
stk.peekMax(); // return 5, [5, **1**] the stack did not change.
stk.pop();     // return 1, [**5**] the top of the stack and the max element is now 5.
stk.top();     // return 5, [**5**] the stack did not change.

Constraints:

• -107 <= x <= 107

• At most 104 calls will be made to push, pop, top, peekMax, and popMax.

• There will be at least one element in the stack when pop, top, peekMax, or popMax is called.

Follow up: Could you come up with a solution that supports O(1) for each top call and O(logn) for each other call?

ac