class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(root == nullptr) return nullptr;
        swap(root->left, root->right);
        invertTree(root->left);
        invertTree(root->right);
        return root;
    }
};

class Solution {
public:
 TreeNode* invertTree(TreeNode* root) {
        if(root == nullptr) return nullptr;
        stack<TreeNode*> sta;
        sta.push(root);
        while(!sta.empty()){
            TreeNode* node = sta.top();
            sta.pop();
            swap(node->left, node->right);
            if(node->right != nullptr) sta.push(node->right);
            if(node->left != nullptr) sta.push(node->left);
        }
        return root;
    }
};