added convert bst to greater tree

Author: saihariG

Trees/BSTtoGreaterTree.java

@@ -0,0 +1,50 @@
+package Trees;
+
+import java.util.Stack;
+
+// LeetCode 538.
+public class BSTtoGreaterTree {
+    static class TreeNode {
+        int val;
+        TreeNode left,right;
+    }
+    public TreeNode convertBST(TreeNode root) {
+
+        int sum = 0;
+        TreeNode node = root;
+        Stack<TreeNode> stack = new Stack<>();
+
+        while(!stack.isEmpty() || node != null) {
+
+            // push all the nodes in the right subtree
+            while(node != null) {
+                stack.push(node);
+                node = node.right;
+            }
+
+            node = stack.pop();
+            sum += node.val;
+            node.val = sum;
+
+            node = node.left;
+
+        }
+
+        return root;
+    }
+}
+
+
+/* Recursive approach
+class Solution {
+    int sum = 0;
+    public TreeNode convertBST(TreeNode root) {
+        if(root == null) return null;
+
+        convertBST(root.right);
+        sum = sum + root.val;
+        root.val = sum;
+        convertBST(root.left);
+        return root;
+    }
+} */