LeetCode-in-All

124. Binary Tree Maximum Path Sum

Hard

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.

The path sum of a path is the sum of the node’s values in the path.

Given the root of a binary tree, return the maximum path sum of any non-empty path.

Example 1:

Input: root = [1,2,3]

Output: 6

Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.

Example 2:

Input: root = [-10,9,20,null,null,15,7]

Output: 42

Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.

Constraints:

Solution

import com_github_leetcode.TreeNode

/*
 * Definition for a binary tree node.
 * class TreeNode(_value: Int = 0, _left: TreeNode = null, _right: TreeNode = null) {
 *   var value: Int = _value
 *   var left: TreeNode = _left
 *   var right: TreeNode = _right
 * }
 */
object Solution {
    def maxPathSum(root: TreeNode): Int = {
        var max = Int.MinValue

        def helper(node: TreeNode): Int = {
            if (node == null) {
                return 0
            }

            // To avoid negative values on the left side, we compare them with 0.
            val left = math.max(0, helper(node.left))
            val right = math.max(0, helper(node.right))
            val current = node.value + left + right

            if (current > max) {
                max = current
            }

            node.value + math.max(left, right)
        }

        helper(root)
        max
    }
}