Leetcode：剑指 Offer 30. 包含min函数的栈

class MinStack {

    /** initialize your data structure here. */
    private Stack<Integer> minStack;
    private Stack<Integer> helpStack;

    public MinStack() {
        minStack=new Stack<Integer>();
        helpStack=new Stack<Integer>();
    }
    
    public void push(int x) {
        //这里要把与当前最小值相等的新数据也加进去！
        if(helpStack.isEmpty()||x<=helpStack.peek()){
            helpStack.add(x);
        }
        minStack.add(x);   
    }
    
    public void pop() {
        if(minStack.pop().equals(helpStack.peek())){
            helpStack.pop();
        }
    }
    
    public int top() {
        return minStack.peek();
    }
    
    public int min() {
        return helpStack.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();
 */