LeetCode -- Min Stack

Question:

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.

Analysis:

设计一个栈，它能够支持push, pop, top操作，并且能够在常数时间内检索到最小的元素。

solution1: 利用Java内置的Stack类，并且设置一个格外的栈，当每次有元素进栈时都判断是否是额外栈的最小元素。这样当求最小元素时直接peek额外栈的栈顶元素即可。

solution2: 不适用Java内置的Stack类，每个栈节点维护一个最小值（是当前节点至栈底的最小值），其余操作与上面相同，只不过push，pop等基本操作要自己简单的实现一下。

Answer:

Solution1：

class MinStack {
    private Stack<Integer> stack = new Stack<Integer>();
    private Stack<Integer> minStack = new Stack<Integer>();
    public void push(int x) {
        if(minStack.isEmpty() || x <= minStack.peek()) //if x is the minimum value
            minStack.push(x);
        stack.push(x);
    }

    public void pop() {
        if(stack.peek().equals(minStack.peek())) //if the pop value is the minimum
            minStack.pop();
        stack.pop();    
    }

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

     public int getMin() {
         return minStack.peek();
     }
}

Solution2：

class MinStack {
   StackNode top = null;
    public void push(int x) {
        if(top == null) {
                top = new StackNode(x);
                top.min = x;
                top.next = null;
        }
        else {
                StackNode node = new StackNode(x);
                if(x < top.min)
                    node.min = x;
                else node.min = top.min;
                node.next = top;
                top = node;
        }
    }

    public void pop() {
        if(top != null)
                top = top.next;
    }

    public int top() {
        if(top != null)
                return top.val;
        else return 0;
    }

    public int getMin() {
        if(top != null)
                return top.min;
        else return 0;
    } 
}
class StackNode {
    int val;
    int min; //store the min utile this node 
    StackNode next;
    public StackNode(int x) {
        val = x;
    }
}