0101. Symmetric Tree

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

Description

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Example 1:

**Input:** root = [1,2,2,3,4,4,3]
**Output:** true

Example 2:

**Input:** root = [1,2,2,null,3,null,3]
**Output:** false

Constraints:

• The number of nodes in the tree is in the range [1, 1000].

• -100 <= Node.val <= 100

Follow up: Could you solve it both recursively and iteratively?

ac

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return helper(root, root);
    }
    private boolean helper(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) return true;
        if (t1 == null || t2 == null) return false;
        if (t1.val != t2.val) return false;
        return helper(t1.left, t2.right) && helper(t1.right, t2.left);
    } 
}