Java doesn't allow to modify passed-in parameters. For this problem, we need to keep track two things during recursions: the max subpath sum and the current max sum for a given node. One way is to let the helper method return an array of two items; Another way is to pass in the current max as an array of one item.

Binary Tree Maximum Path Sum.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxPathSum(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<Integer> list=new ArrayList<Integer>();
        list.add(Integer.MIN_VALUE); 
        find(root, list); 
        return list.get(0);
    }
    int find(TreeNode root, ArrayList<Integer> list)
    {
        if(root==null) return 0; 
        int left=find(root.left, list); 
        int right=find(root.right, list);
        int total=Math.max(root.val, Math.max(root.val+Math.max(left, right), root.val+left+right));
        list.add(0, Math.max(list.get(0), total)); 
        return Math.max(root.val, root.val+Math.max(left, right));
    }
}