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 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

递归

    int re ; 
    public int maxPathSum(TreeNode root) {
        re = Integer.MIN_VALUE;
        help(root);
        return re;
    }
    
    private int help(TreeNode root){
        if(root ==null){
            return 0;
        }
        int left = help(root.left);
        left = left>0?left:0;
        int right = help(root.right);
        right = right>0?right:0;
        int v = left+right+root.val;
        re = re>v?re:v;
        return left>right?(left+root.val):(right+root.val);
    }