剑指 Offer 27. 二叉树的镜像 #

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { this.val = x; }
}
class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if (root == null || (root.left == null && root.right == null)) return root;
        TreeNode temp = mirrorTree(root.left);
        root.left = mirrorTree(root.right);
        root.right = temp;
        return root;
    }
}