题目：

题解：

func isSymmetric(root *TreeNode) bool {
    u, v := root, root
    q := []*TreeNode{}
    q = append(q, u)
    q = append(q, v)
    for len(q) > 0 {
        u, v = q[0], q[1]
        q = q[2:]
        if u == nil && v == nil {
            continue
        }
        if u == nil || v == nil {
            return false
        }
        if u.Val != v.Val {
            return false
        }
        q = append(q, u.Left)
        q = append(q, v.Right)
        q = append(q, u.Right)
        q = append(q, v.Left)
    }
    return true
}