Method get_mut

publication/code/chapter08/rbtree.rs:287–300

(&self, key: &K)

    }

    fn get_mut(&self, key: &K) -> Option<&mut V> {
        unsafe {
            let mut node = self.root;
            while !node.is_null() {
                node = match (*node).key.cmp(key) {
                    Less => (*node).right,
                    Equal => return Some(&mut (*node).val),
                    Greater => (*node).left,
                }
            }
        }

        None
    }

    // 数据插入
    fn insert(&mut self, key: K, val: V) {