Invert Binary Tree — Recursive Swap

// C++
TreeNode* invertTree(TreeNode* root) {
    if (!root) return nullptr;
    swap(root->left, root->right);
    invertTree(root->left); invertTree(root->right);
    return root;
}

// Java
public TreeNode invertTree(TreeNode root) {
    if (root == null) return null;
    TreeNode tmp = root.left;
    root.left = root.right; root.right = tmp;
    invertTree(root.left); invertTree(root.right);
    return root;
}

# Python
def invertTree(root):
    if not root: return None
    root.left, root.right = invertTree(root.right), invertTree(root.left)
    return root