1class Solution {2    private int best;3    public int diameterOfBinaryTree(TreeNode root) {4        best = 0;5        height(root);6        return best;7    }8    private int height(TreeNode node) {9        if (node == null) return 0;10        int left = height(node.left);11        int right = height(node.right);12        best = Math.max(best, left + right);13        return Math.max(left, right) + 1;14    }15}