155. 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 on the top of the stack.
• void pop() removes the element on the top of the stack.
• int top(0) 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.

Examle 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:

• -2**31 <= val <= 2**31 -1
• Methods pop, top and getMin operations will always be called on on-empty stacks.
• At most 3 * 10**4 calls will be made to push, pop, top and getMin.

Submission Screenshot