124. Binary Tree Maximum Path Su

Given a 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.

For example:
Given the below binary tree,

   1
  / \
 2   3

Return 6.
参考

代码

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