0124-binary-tree-maximum-path-sum.swift

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init() { self.val = 0; self.left = nil; self.right = nil; }
 *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
 *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
 *         self.val = val
 *         self.left = left
 *         self.right = right
 *     }
 * }
 */
class Solution {
    var globalMax: Int = Int.min
    
    // Return max path that '''ends at''' a particular node
    private func ends(at node: TreeNode?) -> Int {
        // Base case
        guard let node = node else { return 0 }
        
        // Recursive cases
        // MAX with 0, to shorten negative paths
        let leftMaxPath  = max(ends(at: node.left),  0)
        let rightMaxPath = max(ends(at: node.right), 0)
        
        let pathIncludingNode = leftMaxPath + node.val + rightMaxPath
        globalMax = max(globalMax, pathIncludingNode)
        
        let pathEndingAtNode = max(leftMaxPath, rightMaxPath) + node.val
        return pathEndingAtNode
    }
    
    func maxPathSum(_ root: TreeNode?) -> Int {
        ends(at: root)
        return globalMax
    }
}