var levelOrder = function(root) {
    if(!root) return []
    let res = [], 
        queue = [root]
    while(queue.length) {
        let curr = [],
            temp = []
        while(queue.length) {
            let node = queue.shift()
            curr.push(node.val)
            if(node.left) temp.push(node.left)
            if(node.right) temp.push(node.right)
        }
        res.push(curr)
        queue = temp
    }
    return res
};

var levelOrder = function(root) {
    const res = []
    var dep = function (node, depth){
        if(!node) return
        res[depth] = res[depth]||[]
        res[depth].push(node.val)
        dep(node.left, depth + 1)
        dep(node.right, depth + 1)
    }
    dep(root, 0)
    return res
};