102-Binary-Tree-Level-Order-Traversal.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
const levelOrder = (root) => {
    if (!root) return [];

    const queue = [];
    queue.push(root);
    const result = [];

    while (queue.length > 0) {
        const level = [];
        const len = queue.length;

        for (let i = 0; i < len; i++) {
            const node = queue.shift();

            level.push(node.val);
            if (node.left) queue.push(node.left);
            if (node.right) queue.push(node.right);
        }

        result.push(level);
    }

    return result;
};