[LeetCode] Invert Binary Tree

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<TreeNode*> 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 };