Min Stack (LeetCode #155)

Challenge Statement

• 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.
• This challenge corresponds to LeetCode #155.

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.

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

Solution

Below is my solution and some test cases. This solution has a constant time complexity O(1) and a constant space complexity O(1).