Check if Two Trees are Symmetric in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of symmetric check for two trees

What is the output of the following C++ code that checks if two binary trees are symmetric?

DSA C++

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

bool isMirror(TreeNode* t1, TreeNode* t2) {
    if (!t1 && !t2) return true;
    if (!t1 || !t2) return false;
    return (t1->val == t2->val) && isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
}

bool isSymmetric(TreeNode* root) {
    if (!root) return true;
    return isMirror(root->left, root->right);
}

int main() {
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(2);
    root->left->left = new TreeNode(3);
    root->left->right = new TreeNode(4);
    root->right->left = new TreeNode(4);
    root->right->right = new TreeNode(3);

    bool result = isSymmetric(root);
    std::cout << (result ? "true" : "false") << std::endl;
    return 0;
}

A true

B false

C Compilation error

D Runtime error

💡 Hint

Check if the left subtree is a mirror of the right subtree.

The given tree is symmetric because the left subtree is a mirror reflection of the right subtree.

The function isMirror recursively checks this property.

❓ Predict Output

intermediate

Output when trees are not symmetric

What is the output of the following C++ code that checks if two binary trees are symmetric?

DSA C++

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

bool isMirror(TreeNode* t1, TreeNode* t2) {
    if (!t1 && !t2) return true;
    if (!t1 || !t2) return false;
    return (t1->val == t2->val) && isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
}

bool isSymmetric(TreeNode* root) {
    if (!root) return true;
    return isMirror(root->left, root->right);
}

int main() {
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(2);
    root->left->right = new TreeNode(3);
    root->right->right = new TreeNode(3);

    bool result = isSymmetric(root);
    std::cout << (result ? "true" : "false") << std::endl;
    return 0;
}

A false

B true

C Compilation error

D Runtime error

💡 Hint

Check if the left subtree is a mirror of the right subtree.

The tree is not symmetric because the left and right subtrees do not mirror each other.

🔧 Debug

advanced

Identify the error in symmetric tree check

What error does the following C++ code produce when checking if two trees are symmetric?

DSA C++

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

bool isMirror(TreeNode* t1, TreeNode* t2) {
    if (!t1 && !t2) return true;
    if (!t1 || !t2) return false;
    return (t1->val == t2->val) && isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
}

bool isSymmetric(TreeNode* root) {
    if (!root) return true;
    return isMirror(root->left, root->right);
}

int main() {
    TreeNode* root = nullptr;
    bool result = isSymmetric(root);
    std::cout << (result ? "true" : "false") << std::endl;
    return 0;
}

A Outputs 'false'

B Segmentation fault (runtime error)

C Compilation error

D Outputs 'true'

💡 Hint

Check if the root is null before accessing its children.

The function isSymmetric does not check if root is null before accessing root->left and root->right.

This causes a segmentation fault at runtime.

🧠 Conceptual

advanced

Understanding symmetry in binary trees

Which of the following statements about symmetric binary trees is TRUE?

A A tree is symmetric if all nodes have the same value.

B A tree is symmetric if its left subtree is identical to its right subtree.

C A tree is symmetric if its left subtree is a mirror reflection of its right subtree.

D A tree is symmetric if it has the same number of nodes on left and right subtrees.

💡 Hint

Symmetry means mirror image, not identical structure.

Symmetry in trees means the left subtree is a mirror reflection of the right subtree, not necessarily identical.

❓ Predict Output

expert

Output of complex symmetric tree check with null nodes

What is the output of the following C++ code that checks if two binary trees are symmetric?

DSA C++

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

bool isMirror(TreeNode* t1, TreeNode* t2) {
    if (!t1 && !t2) return true;
    if (!t1 || !t2) return false;
    return (t1->val == t2->val) && isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
}

bool isSymmetric(TreeNode* root) {
    if (!root) return true;
    return isMirror(root->left, root->right);
}

int main() {
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(2);
    root->left->left = nullptr;
    root->left->right = new TreeNode(3);
    root->right->left = nullptr;
    root->right->right = new TreeNode(3);

    bool result = isSymmetric(root);
    std::cout << (result ? "true" : "false") << std::endl;
    return 0;
}

A true

B Runtime error

C Compilation error

D false

💡 Hint

Check if the left subtree is a mirror of the right subtree including null nodes.

The tree is not symmetric because the left subtree's left child is null but the right subtree's left child is also null, but the right subtree's right child is not null, so the mirror condition fails.