155 Min Stack 最小棧

設計一個支援 push，pop，top 操作，并能在常量時間内檢索最小元素的棧。

    push(x) -- 将元素x推入棧中。

    pop() -- 删除棧頂的元素。

    top() -- 擷取棧頂元素。

    getMin() -- 檢索棧中的最小元素。

示例:

MinStack minStack = new MinStack();

minStack.push(-2);

minStack.push(0);

minStack.push(-3);

minStack.getMin();   --> 傳回 -3.

minStack.pop();

minStack.top();      --> 傳回 0.

minStack.getMin();   --> 傳回 -2.

詳見：https://leetcode.com/problems/min-stack/description/

Java實作：

方法一：

class MinStack {
    private Stack<Integer> stk;
    private Stack<Integer> minStk;

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

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

    public void pop() {
        int top=stk.pop();
        if(minStk.peek()==top){
            minStk.pop();
        }
    }

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

    public int getMin() {
        return minStk.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();
 */      

方法二：

class MinStack {
    private Stack<Integer> stk;
    private Stack<Integer> minStk;

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

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

    public void pop() {
        stk.pop();
        minStk.pop();
    }

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

    public int getMin() {
        return minStk.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();
 */