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155. Min Stack

Easy

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.

Example 1:

Input

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

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

Explanation:

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 * 104 calls will be made to push, pop, top, and getMin.

Solution

# @param {String} string
# @return {String}
class MinStack
  class Node
    attr_accessor :min, :data, :previous_node, :next_node

    def initialize(min, data, previous_node, next_node)
      @min = min
      @data = data
      @previous_node = previous_node
      @next_node = next_node
    end
  end

  def initialize
    # no initialization needed.
  end

=begin
    :type val: Integer
    :rtype: Void
=end
  def push(val)
    if @current_node.nil?
      @current_node = Node.new(val, val, nil, nil)
    else
      @current_node.next_node = Node.new([@current_node.min, val].min, val, @current_node, nil)
      @current_node = @current_node.next_node
    end
  end

=begin
    :rtype: Integer
=end
  def pop
    @current_node = @current_node.previous_node
  end

  def top
    @current_node.data
  end

=begin
    :rtype: Integer
=end
  def get_min
    @current_node.min
  end
end

# Your MinStack object will be instantiated and called as such:
# obj = MinStack.new()
# obj.push(val)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.get_min()

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