【Leetcode】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.

Java: 1.不用内置stack 建了一个node class代替： http://blog.csdn.net/ljiabin/article/details/40982153

class MinStack {
    Node top = null;

    public void push(int x) {
        if (top == null) {
            top = new Node(x);
            top.min = x;
        } else {
            Node temp = new Node(x);
            temp.next = top;
            top = temp;
            top.min = Math.min(top.next.min, x);
        }
    }

    public void pop() {
        top = top.next;
        return;
    }

    public int top() {
        return top == null ? 0 : top.val;
    }

    public int getMin() {
        return top == null ? 0 : top.min;
    }
}

class Node {
    int val;
    int min;
    Node next;

    public Node(int val) {
        this.val = val;
    }
}
           

2.内置stack: http://blog.csdn.net/linhuanmars/article/details/41008731

class MinStack {
    ArrayList<Integer> stack = new ArrayList<Integer>();
    ArrayList<Integer> minStack = new ArrayList<Integer>();
    public void push(int x) {
        stack.add(x);
        if(minStack.isEmpty() || minStack.get(minStack.size()-1)>=x)
        {
            minStack.add(x);
        }
    }

    public void pop() {
        if(stack.isEmpty())
        {
            return;
        }
        int elem = stack.remove(stack.size()-1);
        if(!minStack.isEmpty() && elem == minStack.get(minStack.size()-1))
        {
            minStack.remove(minStack.size()-1);
        }
    }

    public int top() {
        if(!stack.isEmpty())
            return stack.get(stack.size()-1);
        return 0;
    }

    public int getMin() {
        if(!minStack.isEmpty())
            return minStack.get(minStack.size()-1);
        return 0;
    }
}