[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:

Swap the

left

subtree and the

right

subtree;

Invert the

left

subtree;

Invert the

right

subtree.

So we immediately have the following simple recursive solution.

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

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.

class Solution {
public:
TreeNode* invertTree(TreeNode* root) {
if (!root) return NULL;
queue<TreeNode*> level;
level.push(root);
while (!level.empty()) {
TreeNode* node = level.front();
level.pop();
swap(node -> left, node -> right);
if (node -> left) level.push(node -> left);
if (node -> right) level.push(node -> right);
}
return root;
}
};