• 因为要在常量时间内查询出最小值，所以需要有另外的数据结构维护最小值，很自然地想到了“堆”这个结构，“最小堆”的堆顶元素刚好是最小值
• 因此出栈和入栈的同时也要维护好最小堆

class MinStack {PriorityQueue<Integer> heap;LinkedList<Integer> stack;public MinStack() {this.heap=new PriorityQueue<>();this.stack=new LinkedList<>();}public void push(int val) { // 入栈heap.offer(val);stack.offer(val);}public void pop() { // 出栈Integer peek = stack.pollLast();heap.remove(peek);}public int top() {return stack.getLast();}public int getMin() {return heap.peek();}
}/*** Your MinStack object will be instantiated and called as such:* MinStack obj = new MinStack();* obj.push(val);* obj.pop();* int param_3 = obj.top();* int param_4 = obj.getMin();*/