0101. Symmetric Tree.md

/**
 * 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) {
        function<bool(TreeNode*, TreeNode*)> isSameTree = [&](TreeNode* p, TreeNode* q) -> bool {
            if(!p && !q) return true;
            if(!p || !q) return false;
            if(p->val != q->val) return false;
            return isSameTree(p->left, q->right) && isSameTree(p->right, q->left);
        };

        return isSameTree(root->left, root->right);
    }
};

Time Complexity: O(n), where n is the number of nodes, since we compare each pair of nodes at most once.

Space Complexity: O(h) for the recursion stack, where h is the height of the tree (O(log n) for a balanced tree, O(n) for a skewed tree).