[LeetCode] Invert Binary Tree

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

to

     4
   /   
  7     2
 /    / 
9   6 3   1

递归实现：

    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr)
            return nullptr;

        TreeNode *tmp = root->left;
        root->left = root->right;
        root->right = tmp;
        
        invertTree(root->left);
        invertTree(root->right);
        
        return root;
    }

非递归实现：

    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr)
            return nullptr;

        std::stack<TreeNode*> nodes;
        nodes.push(root);

        while(!nodes.empty()) {
            TreeNode *pNode = nodes.top();
            nodes.pop();
            
            TreeNode *tmp = pNode->left;
            pNode->left = pNode->right;
            pNode->right = tmp;
            
            if (pNode->left)
                nodes.push(pNode->left);
            if (pNode->right)
                nodes.push(pNode->right);
        }
        return root;
    }