Add Maximum Path Sum

Author: marttp

kotlin/Trees/MaximumPathSum.kt

@@ -0,0 +1,36 @@
+import ds.TreeNode
+
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun maxPathSum(root: TreeNode?): Int {
+    // In Kotlin, global variable is bad practice.
+    // So, we use a mutable list to store the max sum and access it by reference.
+    var maxSum = mutableListOf(Int.MIN_VALUE)
+    maxPathSumHelper(root, maxSum)
+    return maxSum[0]
+}
+
+fun maxPathSumHelper(node: TreeNode?, maxSum: MutableList<Int>): Int {
+    // Base case: null nodes have no path sum.
+    if (node == null) {
+        return 0
+    }
+    // Collect the maximum gain we can attain from the left and right
+    // subtrees, setting them to 0 if they're negative.
+    val leftSum = Math.max(maxPathSumHelper(node.left, maxSum), 0)
+    val rightSum = Math.max(maxPathSumHelper(node.right, maxSum), 0)
+    // Update the overall maximum path sum if the current path sum is
+    // larger.
+    maxSum[0] = Math.max(maxSum[0], node.value + leftSum + rightSum)
+    // Return the maximum sum of a single, continuous path with the
+    // current node as an endpoint.
+    return node.value + Math.max(leftSum, rightSum)
+}