Merge pull request neetcode-gh#1134 from Mahim1997/dev/124_swift

124. Binary Tree Maximum Path Sum

Author: Ahmad-A0

swift/124-Binary-Tree-Maximum-Path-Sum.swift

@@ -0,0 +1,40 @@
+/**
+ * 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
+    }
+}