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.

思路：可以用两个stack完成，但是下面这个方法也不错，用一个linked node list就可以了。

class MinStack {public:    struct Node    {        int min;        int val;        Node *next;        Node(int x):val(x), next(NULL){}    };    MinStack() {        topNode = NULL;    }    void push(int x) {        if(topNode == NULL)        {            topNode = new Node(x);            topNode->min = x;        }        else        {            Node* tmp = new Node(x);            tmp->next = topNode;            topNode = tmp;            topNode->min = min(topNode->next->min, x);        }    }    void pop() {        topNode = topNode->next;    }    int top() {        return topNode->val;    }    int getMin() {        return topNode->min;    }    private:        Node* topNode;};/** * 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(); */