C语言

#include "stdbool.h"
#define NULL ((void *)0)

//Definition for a binary tree node.
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
};

bool check(struct TreeNode* p, struct TreeNode* q) {
    if (!p && !q) return true;
    if (!p || !q) return false;
    return p->val == q->val && check(p->left, q->right) && check(p->right, q->left);
}

bool isSymmetric(struct TreeNode* root) {
    return check(root, root);
}