二叉树层次遍历

// @lc code=start
/**
 * 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) {
    let res = []
    if(!root) return res
    let queue = [root]
    while (queue.length) {
        let arr = []
        let len = queue.length
        for(let i = 0;i<len;i++){
            let node = queue.shift()
            arr.push(node.val)
            node.left&&queue.push(node.left)
            node.right&&queue.push(node.right)
        }
        res.push(arr)
    }
    return res
};