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 [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Approach : Use the simple recursive solution.

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/** * Definition for a binary tree node. * struct TreeNode { * int val; * struct TreeNode *left; * struct TreeNode *right; * }; */ bool checkIfSymmetric(struct TreeNode* n1, struct TreeNode* n2){ if(n1 == NULL && n2 == NULL) return true; if(n1 == NULL || n2 == NULL) return false; if(n1->val == n2->val){ return checkIfSymmetric(n1->left, n2->right) && checkIfSymmetric(n1->right, n2->left); } return false; } bool isSymmetric(struct TreeNode* root) { if(root == NULL){ return true; } return checkIfSymmetric(root->left, root->right); }