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LeetCode 101: Symmetric Tree Solution

Master LeetCode problem 101 (Symmetric Tree), a easy challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

Problem Explanation

Explanation

To check if a binary tree is symmetric, we can recursively compare the left and right subtrees.

  1. Define a recursive helper function that takes in two nodes as arguments.
  2. Check if both nodes are null, return true.
  3. Check if only one of the nodes is null or their values are not equal, return false.
  4. Recursively call the helper function with the left node of the left subtree and the right node of the right subtree, and also with the right node of the left subtree and the left node of the right subtree.
  5. Return the logical AND of both recursive calls.

The time complexity of this approach is O(n) where n is the number of nodes in the tree, as we need to visit each node once. The space complexity is O(n) due to the recursive calls on the stack.

Solution Code

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isMirror(root.left, root.right);
    }
    
    private boolean isMirror(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null || left.val != right.val) {
            return false;
        }
        return isMirror(left.left, right.right) && isMirror(left.right, right.left);
    }
}

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