var levelOrder = function(root) {
    if (root === null) return [];
    
    let result = [[root.val]];
    let parents = [];
    let children = [];
    let tempValue = [];
    
    function hasChildren(node, arr) {
        if(node.left !== null) arr.push(node.left);
        if(node.right !== null) arr.push(node.right);
    }
    
    hasChildren(root, parents);
    
    while(parents.length > 0) {
        let node = parents.shift();
        hasChildren(node, children);
        tempValue.push(node.val);
        if(parents.length === 0) {
            result.push(tempValue);
            tempValue = [];
        }
        if(parents.length == 0 && children.length !== 0) {
            parents = [...children];
            children = [];
        }
    }
    
    return result;
};