给定一个非空二叉树，返回其最大路径和。
本题中，路径被定义为一条从树中任意节点出发，达到任意节点的序列。该路径至少包含一个节点，且不一定经过根节点。
示例 1:
输入: [1,2,3]1/ \2 3输出: 6
示例 2:
输入: [-10,9,20,null,null,15,7]-10/ \9 20/ \15 7输出: 42
递归
/*** Definition for a binary tree node.* public class TreeNode {* int val;* TreeNode left;* TreeNode right;* TreeNode(int x) { val = x; }* }*/
class Solution {private Integer max = Integer.MIN_VALUE;public int maxPathSum(TreeNode root) {if(root == null)return 0;int leftLength = 0, rightLength = 0;if(root.left != null){leftLength = recurise(root.left);}if(root.right != null){rightLength = recurise(root.right);}if(leftLength + rightLength + root.val > max){max = leftLength + rightLength + root.val;}return Math.max(max, Math.max(leftLength + root.val, rightLength + root.val));}public int recurise(TreeNode root) {if(root == null)return 0;int leftLength = 0, rightLength = 0;if(root.left != null){leftLength = recurise(root.left);}if(root.right != null){rightLength = recurise(root.right);}if(leftLength + rightLength + root.val > max){max = leftLength + rightLength + root.val;}return Math.max(0,Math.max(leftLength + root.val, rightLength + root.val));}
}