Min Stack

Two way : Use stack or use linkedlist with node

[x] ##### Min Stack这个题目，用了Node，这个思想很厉害！！！！
[x] ##### 用Node, 通过一个linked list of nodes, 完成remove 操作，用head一直指向栈顶，
[x] ##### 并且用一个min永远保存当前最小的min！！！！
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.
Example:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   -->Returns -3.
minStack.pop();
minStack.top();      ->Returns 0.
minStack.getMin();   -->Returns -2.//-3已经被pop出去了。怎么更新min value in stack
class MinStack {

    /** initialize your data structure here. */
    private Node head;
    public MinStack() {

    }

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

    public void pop() {
        head = head.next;
    }

    public int top() {
        return head.val;
    }

    public int getMin() {
        return head.min;
    }
    private class Node{
        int val; 
        int min;
        Node next;
        private Node(int val, int min) {
            this(val, min, null);
        }
        private Node(int val, int min, Node next) {
            this.val = val;
            this.min = min;
            this.next = next;
        }
    }

}


/**
 * 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();
 */

Stack:

public class MinStack {

    /** initialize your data structure here. */
    /**
     * 155. 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.
     Example:
     MinStack minStack = new MinStack();
     minStack.push(-2);
     minStack.push(0);
     minStack.push(-3);
     minStack.getMin();   --> Returns -3.
     minStack.pop();
     minStack.top();      --> Returns 0.
     minStack.getMin();   --> Returns -2.
     在-128~127的Integer值并且以Integer x = value;的方式赋值的Integer值在进行==和equals比较时，都会返回true，
     因为Java里面对处在在-128~127之间的Integer值，用的是原生数据类型int，会在内存里供重用，
     也就是说这之间的Integer值进行==比较时只是进行int原生数据类型的数值比较，而超出-128~127的范围，进行==比较时是进行地址及数值比较
     time : O(1)
     space : O(n)
     */

    private Stack<Integer> stack;
    private Stack<Integer> minStack;


    public MinStack() {
        stack = new Stack<>();
        minStack = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (!minStack.isEmpty()) {
            int min = minStack.peek();
            if (x <= min) {
                minStack.push(x);
            }
        } else {
            minStack.push(x);
        }
    }

    public void pop() {
        int x = stack.pop();
        if (!minStack.isEmpty()) {
            if (x == minStack.peek()) {
                minStack.pop();
            }
        }
    }

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

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

class MinStack2 {

    /**
     minStack.push(-2);
     minStack.push(0);
     minStack.push(-3);
     minStack.getMin();   --> Returns -3.
     minStack.pop();
     minStack.top();      --> Returns 0.
     minStack.getMin();   --> Returns -2.
     */

    Stack<Integer> stack;
    int min;

    public MinStack2() {
        stack = new Stack<>();
        min = Integer.MAX_VALUE;
    }

    public void push(int x) {
        if (x <= min) {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }

    public void pop() {
        if (stack.pop() == min) {
            min = stack.pop();
        }
    }

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

    public int getMin() {
        return min;
    }
}