看二叉查找树的时候看到put方法：

public void put(Key key, Value val) { if (key == null) throw new NullPointerException("first argument to put() is null"); if (val == null) { delete(key); return; } root = put(root, key, val); assert check(); } private Node put(Node x, Key key, Value val) { //if key存在于以x为结点的子树中则更新它的值 //then 将以key和val为键值的新结点插入到该子树中 if (x == null) return new Node(key, val, 1); int cmp = key.compareTo(x.key); //比它小往左边走 if (cmp < 0) x.left = put(x.left, key, val); //不然往右边走 else if (cmp > 0) x.right = put(x.right, key, val); //找到了就赋新的值 else x.val = val; x.size = 1 + size(x.left) + size(x.right);//为什么要+1？？？ return x; }

为什么要x.size = 1 + size(x.left) + size(x.right);？？进入这里的条件是如果该结已存在就赋予新的值吧？那么仅仅是更新一个值为什么要增加1个size呢？