102. Binary Tree Level Order Traversal

Return the level-order traversal of a binary tree's values: left to right, one level at a time.

Pre-order DFS carrying the depth. Append each value to res[depth], creating that level the first time it is reached.

1function levelOrder(root: TreeNode | null): number[][] {
2  const res: number[][] = [];
3  function dfs(n: TreeNode | null, d: number) {
4    if (!n) return;
5    if (res.length === d) res.push([]);
6    res[d].push(n.val);
7    dfs(n.left, d + 1);
8    dfs(n.right, d + 1);
9  }
10  dfs(root, 0);
11  return res;
12}

tree

result

[]

First node at depth 0: start a new level.

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