题目:

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.

分类:Tree BFS DFS

代码:

 /**
* Definition for a binary tree node.
* 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 helper(root->left, root->right);
} bool helper(TreeNode* p, TreeNode* q) {
if (!p && !q) {
return true;
} else if (!p || !q) {
return false;
} if (p->val != q->val) {
return false;
} return helper(p->left,q->right) && helper(p->right, q->left);
}
};

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