101 对称二叉树

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xn08xg/

https://leetcode.cn/problems/validate-binary-search-tree/description/

cost:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) { return isSymmetric_(root, root); }

private:
    bool isSymmetric_(TreeNode* root, TreeNode* root2) {
        if (root == nullptr && root2 == nullptr) {
            return true;
        }
        if (root == nullptr || root2 == nullptr) {
            return false;
        }
        return root->val == root2->val &&
               isSymmetric_(root->right, root2->left) &&
               isSymmetric_(root->left, root2->right);
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(n)