28. 对称的二叉树

https://www.nowcoder.com/practice/ff05d44dfdb04e1d83bdbdab320efbcb?tpId=13&tqId=11211&tPage=1&rp=1&ru=%2Fta%2Fcoding-interviews&qru=%2Fta%2Fcoding-interviews%2Fquestion-ranking&from=cyc_github&tab=answerKey

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    bool f(TreeNode* p1, TreeNode* p2){
        if(p1 == nullptr && p2 == nullptr) return true;
        if(p1 == nullptr || p2 == nullptr) return false;
        if(p1->val == p2->val){
            return f(p1->left, p2->right) && f(p1->right, p2->left);
        }else{
            return false;
        }
    }
    bool isSymmetrical(TreeNode* pRoot) {
        if(pRoot == nullptr) return true;
        return f(pRoot->left, pRoot->right);
    }

};