/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* helper( TreeNode* root)
    {
        if( ! root)
            return NULL;
        TreeNode* temp  =root->left;
        root-> left = helper( root->right);
        root->right = helper( temp);
        return root;
    }
    TreeNode* invertTree(TreeNode* root) {
        return  helper(root);
        
    }
};