124. Binary Tree Maximum Path Sum (Hard)

https://leetcode.com/problems/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

Hints

Solutions

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int max = Integer.MIN_VALUE;

    public int maxPathSum(TreeNode root) {
        if (root == null) return 0;
        dfs(root);
        return max;
    }

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