226. Invert Binary Tree

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

to

     4
   /   
  7     2
 /    / 
9   6 3   1

  TreeNode* invertTree(TreeNode* root) {
       if(root==NULL)
          return NULL;
       TreeNode* temp;
       temp=root->right;
       root->right=invertTree(root->left);
       root->left=invertTree(temp);
       return root;
    }