Two-Hundred-Sixty Commit: Add Binary Search Tree Iterator problem to Concurrency section

Author: AAdewunmi

src/Concurrency/Problem_3_Binary_Search_Tree_Iterator.java

@@ -0,0 +1,47 @@
+package Concurrency;
+
+// Problem Statement: Binary Search Tree Iterator (medium)
+// LeetCode Question: 173. Binary Search Tree Iterator
+
+import java.util.Stack;
+
+public class Problem_3_Binary_Search_Tree_Iterator {
+
+    class TreeNode {
+       int val;
+       TreeNode left;
+       TreeNode right;
+       public TreeNode(int val) {
+         this.val = val;
+         this.left = null;
+         this.right = null;
+       }
+     }
+
+    private Stack<TreeNode> stack = new Stack<TreeNode>();
+
+    public Problem_3_Binary_Search_Tree_Iterator(TreeNode root) {
+        traverseLeft(root);
+    }
+
+    /** @return whether we have a next smallest number */
+    public boolean hasNext() {
+        return !stack.isEmpty();
+    }
+
+    /** @return the next smallest number */
+    public int next() {
+        TreeNode tmpNode = stack.pop();
+        traverseLeft(tmpNode.right);
+        return tmpNode.val;
+    }
+
+    /** traverse the left sub-tree to push all nodes on the stack */
+    private void traverseLeft(TreeNode node) {
+        while(node != null){
+            stack.push(node);
+            node = node.left;
+        }
+    }
+
+}