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27.二叉树的镜像

题目描述

请完成一个函数,输入一个二叉树,该函数输出它的镜像。
例如输入:
4
/ \
2 7
/ \ / \
1 3 6 9
镜像输出:
4
/ \
7 2
/ \ / \
9 6 3 1
示例 1:
输入:root = [4,2,7,1,3,6,9] 输出:[4,7,2,9,6,3,1]
限制:
0 <= 节点个数 <= 1000
注意:本题与主站 226 题相同:https://leetcode-cn.com/problems/invert-binary-tree/
来源:力扣(LeetCode) 链接:https://leetcode-cn.com/problems/er-cha-shu-de-jing-xiang-lcof 著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。

题解一

递归
func mirrorTree(_ root: TreeNode?) -> TreeNode? {
if root == nil {
return nil
}
let left = root?.left
let right = root?.right
root?.left = mirrorTree(right)
root?.right = mirrorTree(left)
return root
}

题解二

stack
func mirrorTree2(_ root: TreeNode?) -> TreeNode? {
var stack: [TreeNode?] = [root]
while stack.last != nil {
let node = stack.removeLast()
let right = node?.right
let left = node?.left
if let rn = right {
stack.append(rn)
}
if let ln = left {
stack.append(ln)
}
node?.left = right
node?.right = left
}
return root
}