BFS增加order。

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/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var zigzagLevelOrder = function(root) {
    if (!root) {
        return []
    }
    let queue = []
    queue.push(root)
    let order = true
    let res = []
    while (queue.length) {
        let n = queue.length
        let arr = []
        if (order) {
            while (n--) {
                let node = queue.shift()
                arr.push(node.val)
                if (node.left) queue.push(node.left)
                if (node.right) queue.push(node.right)
            }
        } else {
            while (n--) {
                let node = queue.pop()
                arr.push(node.val)
                if (node.right) queue.unshift(node.right)
                if (node.left) queue.unshift(node.left)
            }
        }
        res.push(arr)
        order = !order
    }
    return res
};