[leetCode]155.最小栈

辅助栈

使用辅助栈记录栈每个状态的最小值

class MinStack {
    
    private Stack<Integer> dataStack;
    private Stack<Integer> minStack;

    /** initialize your data structure here. */
    public MinStack() {
        dataStack = new Stack<>();
        minStack = new Stack<>();
    }
    
    public void push(int x) {
       dataStack.push(x);
       if(minStack.isEmpty() || x <= minStack.peek()){
           minStack.push(x);
       }
    }
    
    public void pop() {
        int x = dataStack.pop();
        if( x == minStack.peek()){
            minStack.pop();
        }
    }
    
    public int top() {
        return dataStack.peek();
    }
    
    public int getMin() {
        return minStack.peek();
    }
}

stack

使用自定义数据结构，保存当前元素外还保存最小值

class MinStack {
    
    private Stack<Node> stack;

    class Node{
        int val;
        int min;
        public Node(int x, int m) {
            val = x;
            min = m;
        }
    }
    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
    }
    
    public void push(int x) {
        if(stack.isEmpty()){
            stack.push(new Node(x,x));
        }else {

            stack.push(new Node(x,Math.min(x,stack.peek().min)));
        }
    }
    
    public void pop() {
        stack.pop();
    }
    
    public int top() {
        return stack.peek().val;
    }
    
    public int getMin() {
        return stack.peek().min;
    }
}

自定义栈

使用单链表自定义栈，每个结点保存当前栈的最小值

class MinStack {

    private Node first;

    class Node{
        int val;
        int min;
        Node next;
        public Node(int x, int m, Node n) {
            val = x;
            min = m;
            next = n;
        }
    }
    /** initialize your data structure here. */
    public MinStack() {
       
    }
    
    public void push(int x) {
        Node lastFirst = first;
        if(first == null){
             first = new Node(x, x, lastFirst);
        }else{
            first = new Node(x, 
            Math.min(first.min,x),lastFirst);
        }
       
    }
    
    public void pop() {
        first = first.next;
    }
    
    public int top() {
        return first.val;
    }
    
    public int getMin() {
        return first.min;
    }
}