剑指Offer - 面试题59 - II. 队列的最大值（deque模拟单调栈）

示例 1：
输入: 
["MaxQueue","push_back","push_back","max_value",
"pop_front","max_value"]
[[],[1],[2],[],[],[]]
输出: [null,null,null,2,1,2]

示例 2：
输入: 
["MaxQueue","pop_front","max_value"]
[[],[],[]]
输出: [null,-1,-1]
 
限制：
1 <= push_back,pop_front,max_value的总操作数 <= 10000
1 <= value <= 10^5

class MaxQueue {
	int v;
	queue<int> q;
    deque<int> Maxlist;
public:
    MaxQueue() {}
    
    int max_value() {
    	if(q.empty())
    		return -1;
    	return Maxlist.front();
    }
    
    void push_back(int value) {
    	q.push(value);
    	while(!Maxlist.empty() && Maxlist.back() < value)
    		Maxlist.pop_back();
    	Maxlist.push_back(value);
    }
    
    int pop_front() {
    	if(q.empty())
    		return -1;
    	if(q.front() == Maxlist.front())
    		Maxlist.pop_front();
    	v = q.front();
    	q.pop();
    	return v;
    }
};