Leetcode 0155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. Implement the `MinStack` class: - `MinStack()` initializes the stack object. - `void push(int val)` pushes the element val onto the stack.

Medium

Description

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

• MinStack minStack = new MinStack();
  minStack.push(-2);
  minStack.push(0);
  minStack.push(-3);
  minStack.getMin(); // return -3
  minStack.pop();
  minStack.top(); // return 0
  minStack.getMin(); // return -2

Constraints:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 \* 10^4 calls will be made to push, pop, top, and getMin.

Solution

class MinStack {
    private Stack<Integer> stack = new Stack<>();
    private Stack<Integer> min = new Stack<>();

    public void push(int val) {
      if (min.empty() || getMin() >= val) {
        min.push(val);
      }
      stack.push(val);
    }

    public void pop() {
      if (stack.peek() == getMin()) {
        min.pop();
      }
      stack.pop();
    }

    public int top() {
      return stack.peek();
    }

    public int getMin() {
      return min.peek();
    }
}