543. Diameter of Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int maxLen;
    public int diameterOfBinaryTree(TreeNode root) {
        maxLen = 1;
        diameterOfBinaryTreeHelper(root);
        return maxLen - 1;
    }
    
    public int diameterOfBinaryTreeHelper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int left = diameterOfBinaryTreeHelper(root.left);
        int right = diameterOfBinaryTreeHelper(root.right);
        
        int curMaxLen = left + right + 1;
        maxLen = Math.max(maxLen, curMaxLen);
        return Math.max(left, right) + 1;
    }
}