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.

Tips:

建立递减栈，将小于等于minStack栈顶元素都加入即可。

Code：

public class MinStack {

    /** initialize your data structure here. */
    Stack<Integer> stack;
    Stack<Integer> minStack;
    public MinStack() {
        this.stack = new Stack<>();
        this.minStack = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (minStack.isEmpty() || x <= minStack.peek()) minStack.push(x);
    }

    public void pop() {
        int x = stack.pop();
        if (minStack.peek() == x) minStack.pop();
    }

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

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