public class MaxPathSumBinaryTreeIII {
  public int maxPathSum(TreeNode root) {
    // Assumptions: root is not null.
    int[] max = new int[] { Integer.MIN_VALUE };
    helper(root, max);
    return max[0];
  }

  private int helper(TreeNode root, int[] max) {
    if (root == null) {
      return 0;
    }
    int left = helper(root.left, max);
    int right = helper(root.right, max);
    int sin = Math.max(Math.max(left, right), 0) + root.key;
    max[0] = Math.max(max[0], sin);
    return sin;
  }
}