solution.ts

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 result = -Infinity;
    function postOrder (root:TreeNode|null):number {
        if (root === null) {
            return 0;
        }
        const left = Math.max(postOrder(root.left), 0);
        const right = Math.max(postOrder(root.right), 0);
        result = Math.max(
            result,
            root.val + left + right
        );
        return Math.max(left, right) + root.val;
    }
    postOrder(root);
    return result;
}