problems: add BST iterator

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/binary-search-tree-iterator.java

@@ -0,0 +1,88 @@
+import java.util.ArrayDeque;
+
+/* *
+ * 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 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 BSTIterator {
+    private enum Position {
+        Todo, Done
+    }
+
+    private class Entry {
+        public TreeNode node;
+        public Position position;
+
+        public Entry(TreeNode node, Position position) {
+            this.node = node;
+            this.position = position;
+        }
+
+        public void expand(ArrayDeque<Entry> stack) {
+            if (node.right != null) {
+                stack.addLast(new Entry(node.right, Position.Todo));
+            }
+
+            this.position = Position.Done;
+            stack.addLast(this);
+
+            if (node.left != null) {
+                stack.addLast(new Entry(node.left, Position.Todo));
+            }
+        }
+    }
+
+    private ArrayDeque<Entry> stack = new ArrayDeque<>();
+    public BSTIterator(TreeNode root) {
+        stack.addLast(new Entry(root, Position.Todo));
+    }
+
+    public int next() {
+        for (
+            Entry last = stack.getLast();
+            last != null && last.position == Position.Todo;
+            last = stack.getLast()
+        ) {
+            stack.removeLast().expand(stack);
+        }
+
+        return stack.removeLast().node.val;
+    }
+
+    public boolean hasNext() {
+        Entry last = stack.peekLast();
+        return last != null && (last.position != Position.Todo || last.node != null);
+    }
+}
+
+/**
+ * Your BSTIterator object will be instantiated and called as such:
+ * BSTIterator obj = new BSTIterator(root);
+ * int param_1 = obj.next();
+ * boolean param_2 = obj.hasNext();
+ */