go: add «1038. Binary Search Tree to Greater Sum Tree»

Signed-off-by: Matej Focko <me@mfocko.xyz>

go/binary-search-tree-to-greater-sum-tree.go

@@ -0,0 +1,26 @@
+package main
+
+func bstToGst(root *TreeNode) *TreeNode {
+	var rec func(*TreeNode, int) int
+	rec = func(node *TreeNode, toAdd int) int {
+		if node == nil {
+			return toAdd
+		}
+
+		// recurse to right
+		toAdd = rec(node.Right, toAdd)
+
+		// add to the current value
+		originalValue := node.Val
+		node.Val += toAdd
+
+		// pass new sum to the left
+		toAdd = rec(node.Left, toAdd+originalValue)
+
+		// return new sum
+		return toAdd
+	}
+	rec(root, 0)
+
+	return root
+}