Leetcode sword finger offer 27. image of binary tree

Title Description ：

Please complete a function , Enter a binary tree , This function outputs its image .

For example, the input ：

     4
   /   \
  2     7
 / \   / \
1   3 6   9
airplay mirroring ：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

Examples ：

Example 1：

Input ：root = [4,2,7,1,3,6,9]
Output ：[4,7,2,9,6,3,1]

Limit ：

0 <= Number of nodes <= 1000

Their thinking ：

Binary tree image definition ：

For any node in a binary tree root , Set its left / The right child nodes are left,right; The corresponding in the image of the binary tree root node , To the left / The right child nodes are right,left .

recursive

This is a classic binary tree problem . obviously , We start at the root node , Recursively traverse the tree , And first flip the leaf node to get the image . If the node currently traversed root\textit{root}root Both the left and right subtrees of have been flipped to get a mirror image , Then we just need to exchange the positions of the two subtrees , You can get root\textit{root}root It is a mirror image of the entire subtree of the root node .

Java Program ：

class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode left = mirrorTree(root.left);
        TreeNode right = mirrorTree(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
}