After reading the quote below the problem, you will be motivated to solve it immediately :-) Well, indeed it is also relative easy. The process of inverting a binary tree is simply 3 steps:

1. Swap the left subtree and the right subtree;
2. Invert the left subtree;
3. Invert the right subtree.

So we immediately have the following simple recursive solution.

1 class Solution { 2 public: 3     TreeNode* invertTree(TreeNode* root) { 4         if (!root) return NULL; 5         swap(root -> left, root -> right); 6         root -> left = invertTree(root -> left); 7         root -> right = invertTree(root -> right); 8         return root; 9     }10 };

Well, you may also want to challenge yourself to solve it iteratively. The iterative solution is basically a level-order traversal. Push all the nodes of the same level to a queue and then swap their left subtree and right subtree and iterate over their subtrees.

The code is as follows.

1 class Solution { 2 public: 3     TreeNode* invertTree(TreeNode* root) { 4         if (!root) return NULL; 5         queue
     
       level; 6         level.push(root); 7         while (!level.empty()) { 8             TreeNode* node = level.front(); 9             level.pop();10             swap(node -> left, node -> right);11             if (node -> left) level.push(node -> left);12             if (node -> right) level.push(node -> right);13         }14         return root; 15     }16 };