Method contains

publication/code/chapter08/binary_tree.rs:163–179

查询 key 是否存在于树中

(&self, key: &T)

    // 查询 key 是否存在于树中
    fn contains(&self, key: &T) -> bool {
        match &self.key.cmp(key) {
            Equal => true,
            Greater => {
                match &self.left {
                    Some(left) => left.contains(key),
                    None => false,
                }
            },
            Less => {
                match &self.right {
                    Some(right) => right.contains(key),
                    None => false,
                }
            },
        }
    }

    // 新子节点作为根节点的左子节点
    fn insert_left_tree(&mut self, key: T) {