优先广度优先遍历
    遍历过程中，记录每个每个节点的层级，并将其添加到不同的数组中去

    1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number[][]}
    12.  */
    13. var levelOrder = function(root) {
    14.     if(!root) return [];
    15.     const q = [[root, 0]];
    16.     const res = [];
    17.     while(q.length) {
    18.         const [n, level] = q.shift();
    19.         if(!res[level]) {
    20.             res[level] = [n.val];
    21.         } else {
    22.             res[level].push(n.val);
    23.         }
    24.         if(n.left) q.push([n.left, level + 1]);
    25.         if(n.right) q.push([n.right, level + 1]);
    26.     }
    27.     return res;
    28. };

    优化空间：将队列中属于同一层级的数据，一次性的添加到数组中。

    1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number[][]}
    12.  */
    13. var levelOrder = function(root) {
    14.     if(!root) return [];
    15.     const q = [root];
    16.     const res = [];
    17.     while(q.length) {
    18.         res.push([])
    19.         let length = q.length;
    20.         while(length--) {
    21.             const n = q.shift();
    22.             res[res.length-1].push(n.val)
    23.             if(n.left) q.push(n.left);
    24.             if(n.right) q.push(n.right);
    25.         }
    27.     }
    28.     return res;
    29. };