题目
和求二叉树直径相同套路

class Solution {private int max = Integer.MIN_VALUE;public int maxPathSum(TreeNode root) {if (root == null) {return 0;}dfs(root);return max;}// 返回经过root的单边分支最大和public int dfs(TreeNode root) {if (root == null) {return 0;}int leftMax = Math.max(dfs(root.left), 0);int rightMax = Math.max(dfs(root.right), 0);max = Math.max(max, root.val + leftMax + rightMax);return root.val + Math.max(leftMax, rightMax);}
}