102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree `[3,9,20,null,null,15,7]`,

```    3
/ \
9  20
/  \
15   7
```

return its level order traversal as:

```[
[3],
[9,20],
[15,7]
]
```

102. Binary Tree Level Order Traversal
``````struct Solution;
use rustgym_util::*;

trait Preorder {
fn preorder(&self, levels: &mut Vec<Vec<i32>>, level: usize);
}

impl Preorder for TreeLink {
fn preorder(&self, levels: &mut Vec<Vec<i32>>, level: usize) {
if let Some(node) = self {
let val = node.borrow().val;
if levels.len() == level {
levels.push(vec![val]);
} else {
levels[level].push(val);
}
node.borrow().left.preorder(levels, level + 1);
node.borrow().right.preorder(levels, level + 1);
}
}
}

impl Solution {
fn level_order(root: TreeLink) -> Vec<Vec<i32>> {
let mut res = vec![];
root.preorder(&mut res, 0);
res
}
}

#[test]
fn test() {
let root = tree!(3, tree!(9), tree!(20, tree!(15), tree!(7)));
let res: Vec<Vec<i32>> = vec_vec_i32![[3], [9, 20], [15, 7]];
assert_eq!(Solution::level_order(root), res);
}
``````