2021-12-12：翻转二叉树。力扣226。

package main

import "fmt"

func main() {
    root := &TreeNode{val: 1}
    root.left = &TreeNode{val: 2}
    root.right = &TreeNode{val: 3}
    ret := invertTree(root)
    fmt.Println(ret.val)
    fmt.Println(ret.left.val)
    fmt.Println(ret.right.val)
}

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

func invertTree(root *TreeNode) *TreeNode {
    if root == nil {
        return nil
    }
    left := root.left
    root.left = invertTree(root.right)
    root.right = invertTree(left)
    return root
}