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func zigzagLevelOrder(root *TreeNode) (ans [][]int) {
    if root == nil {
        return
    }
    queue := []*TreeNode{root}
    for level := 0; len(queue) > 0; level++ {
        vals := []int{}
        q := queue
        queue = nil
        for _, node := range q {
            vals = append(vals, node.Val)
            if node.Left != nil {
                queue = append(queue, node.Left)
            }
            if node.Right != nil {
                queue = append(queue, node.Right)
            }
        }
        // 本质上和层序遍历一样，我们只需要把奇数层的元素翻转即可
        if level%2 == 1 {
            for i, n := 0, len(vals); i < n/2; i++ {
                vals[i], vals[n-1-i] = vals[n-1-i], vals[i]
            }
        }
        ans = append(ans, vals)
    }
    return
}