剑指Offer-1.18-30

class MinStack {
    
    

    private Stack<Integer> dataStack;
    private Stack<Integer> minStack;

    /** initialize your data structure here. */
    public MinStack() {
    
    
        dataStack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }
    
    public void push(int x) {
    
    
        dataStack.push(x);
        if (!minStack.isEmpty()) {
    
    
            if (x < minStack.peek()) {
    
    
                minStack.push(x);
            } else {
    
    
                minStack.push(minStack.peek());
            }
        } else {
    
    
            minStack.push(x);
        }
    }
    
    public void pop() {
    
    
        dataStack.pop();
        minStack.pop();
    }
    
    public int top() {
    
    
        if (dataStack != null) {
    
    
            return dataStack.peek();
        } 
        return -1;
    }
    
    public int min() {
    
    
        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.min();
 */