递归法
/**
* 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) {
return !root || dfs(root->left, root->right);
}
bool dfs(TreeNode* p, TreeNode* q) {
if (!p || !q) return !p && !q;
if (p->val != q->val) return false;
return dfs(p->left, q->right) && dfs(p->right, q->left);
}
};