https://leetcode.com/problems/invert-binary-tree/

solution.py

class Solution:
    # @param {TreeNode} root
    # @return {TreeNode}
    def invertTree(self, root):
        if root:
            root.left, root.right = self.invertTree(root.right), self.invertTree(root.left)
        return root

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

    TreeNode swap(TreeNode left, TreeNode right, TreeNode root) {
        root.left = right;
        root.right = left;
        return root;
    }
}