# 101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

``````    1
/ \
2   2
/ \ / \
3  4 4  3
``````

But the following is not:

``````    1
/ \
2   2
\   \
3    3
``````

Note:
Bonus points if you could solve it both recursively and iteratively.

``````class Solution {
public:
bool isSymmetric(TreeNode *root) {
if (!root) return true;
return isSymmetric(root->left, root->right);
}
bool isSymmetric(TreeNode *left, TreeNode *right) {
if (!left && !right) return true;
if (left && !right || !left && right || left->val != right->val) return false;
return isSymmetric(left->left, right->right) && isSymmetric(left->right, right->left);
}

};
``````

``````class Solution {
public:
bool isSymmetric(TreeNode* root) {
if (!root) return true;
queue<TreeNode*> q1, q2;
q1.push(root->left);
q2.push(root->right);
while (!q1.empty() && !q2.empty()) {
TreeNode *node1 = q1.front(); q1.pop();
TreeNode *node2 = q2.front(); q2.pop();
if (!node1 && !node2) continue;
if((node1 && !node2) || (!node1 && node2)) return false;
if (node1->val != node2->val) return false;
q1.push(node1->left);
q1.push(node1->right);
q2.push(node2->right);
q2.push(node2->left);
}
return true;
}
};
``````

https://leetcode.com/problems/symmetric-tree/

https://leetcode.com/problems/symmetric-tree/discuss/33054/Recursive-and-non-recursive-solutions-in-Java

LeetCode All in One 题目讲解汇总(持续更新中…)

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