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class Solution {
public:
    void swap(TreeNode & left, TreeNode *& right)
    {
        TreeNode * tmp = left;
        left = right;
        right = tmp;
    }
    TreeNode * invertTree(TreeNode* root) {
        if (root == NULL)
            return NULL;
        if (root->left != NULL)
            invertTree(root->left);
        if (root->right != NULL)
            invertTree(root->right);
        swap(root->left, root->right);
        return root;
    }
};