102. Binary Tree Level Order Traversal

1/**
2 * Definition for a binary tree node.
3 * class TreeNode {
4 *     val: number
5 *     left: TreeNode | null
6 *     right: TreeNode | null
7 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
8 *         this.val = (val===undefined ? 0 : val)
9 *         this.left = (left===undefined ? null : left)
10 *         this.right = (right===undefined ? null : right)
11 *     }
12 * }
13 */
14
15function levelOrder(root: TreeNode | null): number[][] {
16    if(!root) return []
17
18    let res: number[][] = [];
19    const queue: (TreeNode | null)[] = [root];
20    while (queue.length > 0) {
21        const level_size = queue.length;
22        const curr_level: number[] = [];
23        for (let i = 0; i < level_size; i++) {
24            const node = queue.shift()!;
25            curr_level.push(node.val);
26            if (node.left) queue.push(node.left);
27            if (node.right) queue.push(node.right);
28        }
29        res.push(curr_level)
30    }
31    return res
32};