543. 二叉树的直径

<?php
Class TreeNode {
    /** @var int $val */
    public $val = 0;
    /** @var TreeNode $left */
    public $left;
    /** @var TreeNode $right */
    public $right;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    public $max = 0;
    public function diameterOfBinaryTree($root) {
        if ($root == null) return 0;
        $this->helper($root);
        return $this->max;
    }
    // 取当前节点最大直径
    private function helper($root) {
        $left = ($root->left == null) ? 0 : $this->helper($root->left) + 1;
        $right = ($root->right == null) ? 0 : $this->helper($root->right) + 1;
        $this->max = max($this->max, $left + $right);
        return max($left, $right);
    }
}
/**
 *          4
 *         / \
 *        2   6
 *       / \ /
 *      1  3 5
 *     /
 *    6
 */
$tree = new TreeNode(4);
$tree->left = new TreeNode(2);
$tree->left->left = new TreeNode(1);
$tree->left->left->left = new TreeNode(6);
$tree->left->right = new TreeNode(3);
$tree->right = new TreeNode(6);
$tree->right->left = new TreeNode(5);
$cls = new Solution();
$r = $cls->diameterOfBinaryTree($tree);
print_r($r);