[LeetCode 124] Binary Tree Maximum Path Sum

Problem

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:

- The number of nodes in the tree is in the range [1, 3 * 104].
- -1000 <= Node.val <= 1000

TypeScript

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function maxPathSum(root: TreeNode | null): number {
    let res: number = -Infinity;
    
    const nodeSum = (node: TreeNode): number => {
        if(node === null) return 0;
        
        const leftSum: number = Math.max(nodeSum(node.left), 0)
        const rightSum: number = Math.max(nodeSum(node.right), 0)
        const pathSum = node.val + leftSum + rightSum
        res = Math.max(res, pathSum)
        
        
        return node.val + Math.max(leftSum, rightSum)        
    }
    
    nodeSum(root);
    
    return res;
};

Reference

https://leetcode.com/problems/binary-tree-maximum-path-sum/