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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0101_0200.s0101_symmetric_tree;

// #Easy #Top_100_Liked_Questions #Top_Interview_Questions #Depth_First_Search #Breadth_First_Search
// #Tree #Binary_Tree #Data_Structure_I_Day_11_Tree #Level_2_Day_15_Tree
// #Big_O_Time_O(N)_Space_O(log(N)) #2022_06_22_Time_0_ms_(100.00%)_Space_42.3_MB_(46.67%)

import com_github_leetcode.TreeNode;

/*
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
/**
 * 101 - Symmetric Tree\.
 *
 * Easy
 *
 * Given the `root` of a binary tree, _check whether it is a mirror of itself_ (i.e., symmetric around its center).
 *
 * **Example 1:**
 *
 * ![](https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg)
 *
 * **Input:** root = [1,2,2,3,4,4,3]
 *
 * **Output:** true 
 *
 * **Example 2:**
 *
 * ![](https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg)
 *
 * **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?
**/
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return helper(root.left, root.right);
    }

    private boolean helper(TreeNode leftNode, TreeNode rightNode) {
        if (leftNode == null || rightNode == null) {
            return leftNode == null && rightNode == null;
        }
        if (leftNode.val != rightNode.val) {
            return false;
        }
        return helper(leftNode.left, rightNode.right) && helper(leftNode.right, rightNode.left);
    }
}