class Solution {
    int max = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        maxDepth(root);
        return max;
    }
    public int maxDepth(TreeNode root){
        if(root == null) return 0;
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        max = Math.max(max, left + right);
        return Math.max(left, right) + 1;
    }
}