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:
- 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?
Solution
/**
* Definition for a binary tree node.
* class TreeNode {
* public $val = null;
* public $left = null;
* public $right = null;
* function __construct($val = 0, $left = null, $right = null) {
* $this->val = $val;
* $this->left = $left;
* $this->right = $right;
* }
* }
*/
class Solution {
/**
* @param TreeNode $root
* @return Boolean
*/
function isSymmetric($root) {
if ($root == null) {
return true;
}
return $this->helper($root->left, $root->right);
}
private function helper($leftNode, $rightNode) {
if ($leftNode == null || $rightNode == null) {
return $leftNode == null && $rightNode == null;
}
if ($leftNode->val != $rightNode->val) {
return false;
}
return $this->helper($leftNode->left, $rightNode->right) && $this->helper($leftNode->right, $rightNode->left);
}
}