对称二叉树。给定一个二叉树，检查它是否是镜像对称的。例如，二叉树 [1,2,2,3,4,4,3] 是对称的。力扣101。

递归。head1的左节点 ==head2的右节点。head1的右节点 ==head2的左节点。

代码用golang编写。代码如下：

package main

import "fmt"

func main() {
    head := &TreeNode{}
    head.val = 1

    head.left = &TreeNode{}
    head.left.val = 2

    head.right = &TreeNode{}
    head.right.val = 2
    head.left.left = &TreeNode{}
    head.left.left.val = 3
    head.right.right = &TreeNode{}
    head.right.right.val = 3

    ret := isSymmetric(head)
    fmt.Println(ret)
}

type TreeNode struct {
    val   int
    left  *TreeNode
    right *TreeNode
}

func isSymmetric(root *TreeNode) bool {
    return isMirror(root, root)
}

// 一棵树是原始树  head1
// 另一棵是翻面树  head2
func isMirror(head1 *TreeNode, head2 *TreeNode) bool {
    if head1 == nil && head2 == nil {
        return true
    }
    if head1 != nil && head2 != nil {
        return head1.val == head2.val && isMirror(head1.left, head2.right) && isMirror(head1.right, head2.left)
    }
    // 一个为空，一个不为空  false
    return false
}

执行结果如下：