LeetCode-in-All

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:

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:

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

Solution

import com_github_leetcode.TreeNode

/*
 * Definition for a binary tree node.
 * class TreeNode(_value: Int = 0, _left: TreeNode = null, _right: TreeNode = null) {
 *   var value: Int = _value
 *   var left: TreeNode = _left
 *   var right: TreeNode = _right
 * }
 */
object Solution {
    def isSymmetric(root: TreeNode): Boolean = {
        if (root == null) {
            true
        } else {
            helper(root.left, root.right)
        }
    }

    private def helper(leftNode: TreeNode, rightNode: TreeNode): Boolean = {
        if (leftNode == null || rightNode == null) {
            leftNode == null && rightNode == null
        } else if (leftNode.value != rightNode.value) {
            false
        } else {
            helper(leftNode.left, rightNode.right) && helper(leftNode.right, rightNode.left)
        }
    }
}