二叉树锯齿形层序遍历

var zigzagLevelOrder = function(root) {
    if (!root) {
        return [];
    }
    const ans = [];
    const nodeQueue = [root];
    let isOrderLeft = true;
    while (nodeQueue.length) {
        let levelList = [];
        const size = nodeQueue.length;
        for (let i = 0; i < size; ++i) {
            const node = nodeQueue.shift();
            if (isOrderLeft) {
                levelList.push(node.val);
            } else {
                levelList.unshift(node.val);
            }
            if (node.left !== null) {
                nodeQueue.push(node.left);
            }
            if (node.right !== null) {
                nodeQueue.push(node.right);
            }
        }            
        ans.push(levelList);
        isOrderLeft = !isOrderLeft;
    }
    return ans;
};