[leetcode 101] Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

1
/ \
2   2
/ \ / \
3  4 4  3

But the following is not:

1
/ \
2   2
\   \
3    3

Note:

Bonus points if you could solve it both recursively and iteratively.

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isSymmetric(TreeNode *root) {
if (!root) {
return true;
}
return dfs(root->left, root->right);
}
bool dfs(TreeNode *left, TreeNode *right) {
if (!left && !right) {
return true;
} else if (!left || !right) {
return false;
}
return left->val == right->val && dfs(left->left, right->right) && dfs(left->right, right->left);
}
};