---
tags: leetcode
---

# [101. Symmetric Tree](https://leetcode.com/problems/symmetric-tree/)
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# My Solution
## The Key Idea for Solving This Coding Question
## C++ Code

/**
 * 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->left, root->right);
    }

private:
    bool isSymmetric(TreeNode *t1, TreeNode *t2) {
        if (t1 == nullptr && t2 == nullptr) {
            return true;
        }

        if (t1 == nullptr || t2 == nullptr) {
            return false;
        }

        if (t1->val != t2->val) {
            return false;
        }

        return isSymmetric(t1->left, t2->right) && isSymmetric(t1->right, t2->left); 
    }
};

## Time Complexity
$O(n)$
$n$ is the number of nodes in the binary tree referred by `root`.
## Space Complexity
$O(H)$
$H$ is the height of the binary tree referred by `root`.
# Miscellane
<!--
# Test Cases

[1,2,2,3,4,4,3]

[1,2,2,null,3,null,3]

[1,2,2,2,null,2]

[1,2,2,3,4,4,6]

-->