[Function add]: 1. Add leetcode solutions.

Author: Botao Xiao

README.md

@@ -451,6 +451,10 @@
 
 [283. Move Zeroes](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/283.%20Move%20Zeroes.md)
 
+[284. Peeking Iterator](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/284.%20Peeking%20Iterator.md)
+
+[287. Find the Duplicate Number](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/287.%20Find%20the%20Duplicate%20Number.md)
+
 
 ## Algorithm(4th_Edition)
 Reading notes of book Algorithm(4th Algorithm),ISBN: 9787115293800.

leetcode/284. Peeking Iterator.md

@@ -9,8 +9,8 @@ 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. 
+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.
 ```
 
@@ -62,4 +62,40 @@ class PeekingIterator implements Iterator<Integer> {
             return it.hasNext();
 	}
 }
-```
+```
+
+### Second time
+1. I used a queue to save the values that can be gotten from the iterator.
+2. peek means getting the first value in the queue.
+3. next means poll the queue.
+```Java
+// Java Iterator interface reference:
+// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
+class PeekingIterator implements Iterator<Integer> {
+    Iterator<Integer> it = null;
+    LinkedList<Integer> record = null;
+	public PeekingIterator(Iterator<Integer> iterator) {
+	    // initialize any member here.
+	    this.it = iterator;
+        record = new LinkedList<>();
+        while(it.hasNext()){
+            int num = it.next();
+            this.record.add(num);
+        }
+	}
+    // Returns the next element in the iteration without advancing the iterator.
+	public Integer peek() {
+        return record.get(0);
+	}
+	// hasNext() and next() should behave the same as in the Iterator interface.
+	// Override them if needed.
+	@Override
+	public Integer next() {
+	    return record.poll();
+	}
+	@Override
+	public boolean hasNext() {
+	    return this.record.size() > 0;
+	}
+}
+```

leetcode/287. Find the Duplicate Number.md

@@ -51,4 +51,25 @@ class Solution {
         return fast;
     }
 }
-```
+```
+
+### Second time
+1. This question can be compared with the cyclic list question.
+2. The difference between this two question is we use the index as the "pointer" in this question.
+```Java
+class Solution {
+    public int findDuplicate(int[] nums) {
+        int slow = 0, fast = 0;
+        do{
+            slow = nums[slow];
+            fast = nums[nums[fast]];
+        }while(slow != fast);
+        slow = 0;
+        while(slow != fast){
+            slow = nums[slow];
+            fast = nums[fast];
+        }
+        return slow;
+    }
+}
+```