add 284 Peeking Iterator

Medium/PeekingIterator.java

@@ -0,0 +1,62 @@
+import java.util.Iterator;
+
+/**
+ * Given an Iterator class interface with methods: next() and hasNext(), design and implement a PeekingIterator that
+ * support the peek() operation -- it essentially peek() at the element that will be returned by the next call to
+ * next().
+ *
+ * Here is an example. Assume that the iterator is initialized to the beginning of the list: [1, 2, 3].
+ *
+ * Call next() gets you 1, the first element in the list.
+ *
+ * Now you call peek() and it returns 2, the next element. Calling next() after that still return 2.
+ *
+ * You call next() the final time and it returns 3, the last element. Calling hasNext() after that should return false.
+ *
+ * Hint:
+ *
+ * Think of "looking ahead". You want to cache the next element. Is one variable sufficient? Why or why not?
+ *
+ * Test your design with call order of peek() before next() vs next() before peek().
+ *
+ * For a clean implementation, check out Google's guava library source code.
+ *
+ * Follow up: How would you extend your design to be generic and work with all types, not just integer?
+ *
+ * Tags: Design
+ *
+ * Similar Problems: (M) Binary Search Tree Iterator, (M) Flatten 2D Vector, (M) Zigzag Iterator
+ */
+public class PeekingIterator implements Iterator<Integer> {
+
+    private Integer next;
+    private Iterator<Integer> iter;
+
+    public PeekingIterator(Iterator<Integer> iterator) {
+        // initialize any member here.
+        this.iter = iterator;
+        if (iterator.hasNext()) {
+            next = iterator.next();
+        }
+    }
+
+    // Returns the next element in the iteration without advancing the iterator.
+    public Integer peek() {
+        return next;
+    }
+
+    // hasNext() and next() should behave the same as in the Iterator interface.
+    // Override them if needed.
+    @Override
+    public Integer next() {
+        int res = next;
+        next = iter.hasNext() ? iter.next() : null;
+        return res;
+    }
+
+    @Override
+    public boolean hasNext() {
+        return next != null;
+    }
+
+}