155. Min Stack

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

push(x) – Push element x onto stack.
pop() – Removes the element on top of the stack.
top() – Get the top element.
getMin() – Retrieve the minimum element in the stack.
Example:

MinStack minStack = new MinStack();minStack.push(-2);minStack.push(0);minStack.push(-3);minStack.getMin();   --> Returns -3.minStack.pop();minStack.top();      --> Returns 0.minStack.getMin();   --> Returns -2.

class MinStack {            private Stack<Integer> stack = new Stack<Integer>();          private Stack<Integer> minStack = new Stack<Integer>();      public void push(int x) {                if (minStack.isEmpty() || x <= minStack.peek())              minStack.push(x);          stack.push(x);      }      public void pop() {                if (stack.peek().equals(minStack.peek()))              minStack.pop();          stack.pop();      }      public int top() {          return stack.peek();      }      public int getMin() {          return minStack.peek();      }  }  /** * Your MinStack object will be instantiated and called as such: * MinStack obj = new MinStack(); * obj.push(x); * obj.pop(); * int param_3 = obj.top(); * int param_4 = obj.getMin(); */