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[][]}
 */
var levelOrder = function (root) {
    if (!root) return [];

    const result = [];
    const queue = [root];

    while (queue.length) {
        const numNodes = queue.length;
        const temp = [];
        for (let i = 0; i < numNodes; i++) {
            const subtree = queue.shift();
            temp.push(subtree.val);
            if (subtree.left !== null) queue.push(subtree.left);
            if (subtree.right !== null) queue.push(subtree.right);
        }
        result.push(temp);
    }

    return result;
};