[LeetCode]--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.

public class _155MinStack2 {

    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    /** initialize your data structure here. */
    public _155MinStack2() {
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

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

    public void pop() {
        if (!stack.isEmpty()) {
            //判断对象相等用equals
            if (stack.peek().equals(minStack.peek())) {
                minStack.pop();
            }
            stack.pop();
        }
    }

    public int top() {
        if (!stack.isEmpty())
            return stack.peek();
        return 0;
    }

    public int getMin() {
        if (!minStack.isEmpty())
            return minStack.peek();
        return 0;
    }

    /**
     * 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();
     */