[leetcode]124. Binary Tree Maximum Path Sum二叉树最大路径和

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

题意：

从二叉树任意节点出发，到任意节点终止。计算最大路径和。

思路：

路径从任意节点出发，到任意节点终止。

代码：

 1 class Solution {
 2     public int maxPathSum(TreeNode root) {
 3         // corner case 
 4         if(root == null){return 0;}
 5         int[] maxPath = new int[]{Integer.MIN_VALUE};
 6         dfs(root, maxPath);
 7         return maxPath[0];
 8     }
 9     
10     private int dfs(TreeNode root, int[]maxPath){
11         int left = root.left != null ? Math.max(dfs(root.left, maxPath), 0) : 0;
12         int right = root.right != null ? Math.max(dfs(root.right, maxPath), 0) : 0;
13         int cur = root.val + left + right;
14         maxPath[0] = Math.max(maxPath[0], cur);
15         return root.val  + Math.max(left, right);
16     }   
17 }