leetcode-124 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 nodeand 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

 

该题意思为找出一条最大路径：其中改路劲可以从左节点某个位置到达右节点某个位置，其中不能分叉，该情况将其看成一个无向图，那么该题就是找出一个节点，计算出其节点左边的最大路径和右边的最大路径求结果值即可。

     int max = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
        if(root == null) {
            return 0;
        }
         helper(root);
         return max;
    }

    private int helper(TreeNode root){
        if(root == null) {
            return 0;
        }else {
            int left = Math.max(0, helper(root.left));
            int right = Math.max(0,helper(root.right));
            max = Math.max(max, root.val + left + right);
            return Math.max(left+root.val, root.val+right);
            
        }
 
    }