Added Maximum Sum BST in Binary Tree.java

Author: varunu28

Hard/Maximum Sum BST in Binary Tree.java

@@ -0,0 +1,39 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+  public int maxSumBST(TreeNode root) {
+    int[] ans = {0};
+    helper(root, ans);
+    return ans[0];
+  }
+  
+  private int[] helper(TreeNode root, int[] ans) {
+    if (root == null) {
+      // new int[]{min, max, sum}
+      return new int[]{Integer.MAX_VALUE, Integer.MIN_VALUE, 0};
+    }
+    int[] left = helper(root.left, ans);
+    int[] right = helper(root.right, ans);
+    if (!(left != null && right != null && root.val > left[1] && root.val < right[0])) {
+      return null;
+    }
+    int sum = root.val + left[2] + right[2];
+    ans[0] = Math.max(ans[0], sum);
+    int min = Math.min(root.val, left[0]);
+    int max = Math.max(root.val, right[1]);
+    return new int[]{min, max, sum};
+  }
+}