leetcode543 - Diameter of Binary Tree - easy

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int maxLen = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        calHelper(root);
        return maxLen;
    }
    
    /*
    * return the current depth. leaf node has depth of 1.
    */
    private int calHelper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int leftDepth = calHelper(root.left);
        int rightDepth = calHelper(root.right);
        maxLen = Math.max(maxLen, leftDepth + rightDepth);
        return Math.max(leftDepth, rightDepth) + 1;
    }
}