You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: _297_serialize_and_deserialize_binary_tree.rs
    path of file: ./repos/rustgym/leetcode/src/d2
    
    the code of the file until where you have to start completion: use rustgym_util::*;
use std::iter::Peekable;
use std::vec::IntoIter;

struct Codec;

enum Tok {
    Op(char),
    Num(i32),
}

impl Codec {
    fn new() -> Self {
        Codec {}
    }

    fn serialize(&self, root: TreeLink) -> String {
        let mut res = "".to_string();
        Self::serialize_tree(&root, &mut res);
 
    
    fn parse_tokens(data: String) -> Vec<Tok> {
        let mut it = data.chars().peekable();
        let mut res = vec![];
        while let Some(c) = it.next() {
            if c == '(' || c == ')' {
                res.push(Tok::Op(c));
            } else {
                let mut sign = 1;
                let mut x = 0;
                if c == '-' {
                    sign = -1;
                } else {
                    x = (c as u8 - b'0') as i32;
                }
                while let Some('0'..='9') = it.peek() {
                    x *= 10;
                    x += (it.next().unwrap() as u8 - b'0') as i32;
                }
                res.push(Tok::Num(sign * x));
            }
        }
        res
    }