Add 251

Author: ChienkuChen

src/_251/Vector2D.java

@@ -0,0 +1,64 @@
+package _251;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+
+// Need to handle empty List<Integer>
+// For example:
+// [[], [], [1, 2, 3], [4, 5], [], [6], []]
+public class Vector2D implements Iterator<Integer> {
+    private List<List<Integer>> vector;
+    private int rowPointer;
+    private int elementPointer;
+
+    public Vector2D(List<List<Integer>> vec2d) {
+        vector = vec2d;
+        rowPointer = 0;
+        elementPointer = -1;
+    }
+
+    @Override
+    public Integer next() {
+        return vector.get(rowPointer).get(elementPointer);
+    }
+
+    @Override
+    public boolean hasNext() {
+        if (vector.isEmpty())
+            return false;
+
+        if (!vector.get(rowPointer).isEmpty() && (elementPointer + 1) < vector.get(rowPointer).size()) {
+            elementPointer++;
+            return true;
+        }
+
+        if (rowPointer + 1 >= vector.size())
+            return false;
+
+        int tmp = rowPointer + 1;
+        while (tmp < vector.size() && vector.get(tmp).isEmpty())
+            tmp++;
+
+        if (tmp >= vector.size())
+            return false;
+
+        rowPointer = tmp;
+        elementPointer = 0;
+        return true;
+    }
+
+    public static void main(String[] args) {
+        List<List<Integer>> vector = new ArrayList<>();
+        vector.add(Arrays.asList(1, 2));
+        vector.add(Arrays.asList(3));
+        vector.add(Arrays.asList(4, 5, 6));
+        vector.add(new ArrayList<>());
+        vector.add(new ArrayList<>());
+        Vector2D vector2D = new Vector2D(vector);
+
+        while (vector2D.hasNext())
+            System.out.println(vector2D.next());
+    }
+}