class MedianFinder {
    //A为小根堆，B为大根堆
    List<Integer> A,B;
    public MedianFinder() {
        A = new ArrayList<Integer>();
        B = new ArrayList<Integer>();
    }
    
    public void addNum(int num) {
        int m = A.size(),n = B.size();
        if(m == n){
            insert(B,num);
            int top = deleteTop(B);
            insert(A,top);
        }else{
            insert(A,num);
            int top = deleteTop(A);
            insert(B,top);
        }
    }

    //删除堆顶元素并返回其值
    private int deleteTop(List<Integer> list){
        Collections.swap(list,0,list.size() - 1);
        int heapSize = list.size() - 1;
        int root = 0;
        while(root < heapSize/2){
            int left = root * 2 + 1,right = root * 2 + 2,largest = root;
            if(left < heapSize && comparee(list,left,largest))
                largest = left;
            if(right < heapSize &&  comparee(list,right,largest))
                largest = right;
            if(root == largest)
                break;
            Collections.swap(list,root,largest);
            root = largest;
        }
        return list.remove(list.size() - 1);
    }

    //A小根堆的比较方法：比较值<目标值？
    private boolean comparee(List<Integer> list,int source,int target){
        if(list == A)
            return list.get(source) < list.get(target);
        else
            return list.get(source) > list.get(target);
    }

    //将数插入到堆中
    private void insert(List<Integer> list,int num){
        list.add(num);
        int index = list.size() - 1;
        while(index > 0){
            int root = (index - 1) / 2;
            if(!comparee(list,index,root))
                break;
            Collections.swap(list,root,index);
            index = root;
        }
    }
    
    public double findMedian() {
        int m = A.size(),n = B.size();
        return (m==n) ? (A.get(0) + B.get(0)) / 2.0 : A.get(0);
    }
}