Binary Tree Maximum Path Sum

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

1
/ \
2   3

Output: 6

Example 2:

Input: [-10,9,20,null,null,15,7]

-10
/ \
9  20
/  \
15   7

Output: 42

Solution

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
int maxSum = Integer.MIN_VALUE;
public int maxPathSum(TreeNode root) {
if (root == null) return 0;
helper(root);
return maxSum;
}
int helper(TreeNode root) {
if (root == null) return 0;
int leftPath = Math.max(helper(root.left), 0);
int rightPath = Math.max(helper(root.right), 0);

maxSum = Math.max(maxSum, root.val + leftPath + rightPath);
return root.val + Math.max(leftPath, rightPath);
}
}