Implement PeekingIterator for iterator functionality

Author: jyoung2419

leetcode3/정진영/284. Peeking Iterator

@@ -0,0 +1,56 @@
+/**
+ * // This is the Iterator's API interface.
+ * // You should not implement it, or speculate about its implementation.
+ * function Iterator() {
+ *    @ return {number}
+ *    this.next = function() { // return the next number of the iterator
+ *       ...
+ *    }; 
+ *
+ *    @return {boolean}
+ *    this.hasNext = function() { // return true if it still has numbers
+ *       ...
+ *    };
+ * };
+ */
+
+/**
+ * @param {Iterator} iterator
+ */
+var PeekingIterator = function(iterator) {
+    this.iterator = iterator;
+
+    this.nextValue = iterator.hasNext() ? iterator.next() : null;
+};
+
+/**
+ * @return {number}
+ */
+PeekingIterator.prototype.peek = function() {
+    return this.nextValue;
+};
+
+/**
+ * @return {number}
+ */
+PeekingIterator.prototype.next = function() {
+    const ans = this.nextValue;
+
+    this.nextValue = this.iterator.hasNext() ? this.iterator.next() : null;
+    return ans;
+};
+
+/**
+ * @return {boolean}
+ */
+PeekingIterator.prototype.hasNext = function() {
+    return this.nextValue !== null;
+};
+
+/** 
+ * Your PeekingIterator object will be instantiated and called as such:
+ * var obj = new PeekingIterator(arr)
+ * var param_1 = obj.peek()
+ * var param_2 = obj.next()
+ * var param_3 = obj.hasNext()
+ */