add 284 🍺

README.md

@@ -216,6 +216,7 @@ My LeetCode solutions with Chinese explanation. 我的LeetCode中文题解。
 | 278 |[First Bad Version](https://leetcode.com/problems/first-bad-version)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/278.%20First%20Bad%20Version.md)|Easy|  |
 | 279 |[Perfect Squares](https://leetcode.com/problems/perfect-squares/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/279.%20Perfect%20Squares.md)|Medium|  |
 | 283 |[Move Zeroes](https://leetcode.com/problems/move-zeroes)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/283.%20Move%20Zeroes.md)|Easy|  |
+| 284 |[Peeking Iterator](https://leetcode.com/problems/peeking-iterator/)|[C++](solutions/284.%20Peeking%20Iterator.md)|Medium|  |
 | 287 |[Find the Duplicate Number](https://leetcode.com/problems/find-the-duplicate-number/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/287.%20Find%20the%20Duplicate%20Number.md)|Medium|  |
 | 289 |[Game of Life](https://leetcode.com/problems/game-of-life/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/289.%20Game%20of%20Life.md)|Medium|  |
 | 290 |[Word Pattern](https://leetcode.com/problems/word-pattern)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/290.%20Word%20Pattern.md)|Easy|  |

solutions/284. Peeking Iterator.md

@@ -0,0 +1,61 @@
+# [284. Peeking Iterator](https://leetcode.com/problems/peeking-iterator/)
+
+# 思路 
+
+我们有了一个只支持`next`和`hasNext`方法的迭代器类，让我们实现一个子类，要求增添一个`peek`方法。
+
+## 思路一
+我们想实现的`peek`是只返回当前元素而不考虑指针后移。但每次调用基类的`next`时，返回当前元素后都会后移指针。那么我们可以考虑用一个变量`value`在调用基类的`next`时缓存当前元素，这样的话即使指针后移了，我们调用`peek`时也能直接返回value。为此我们需要一个bool变量`flag`来指示`value`是否存放了在之后调用`peek`和`next`应该返回的值。
+
+
+## 思路二
+还有一个比较tricky的方法：`peek`时创建一个当前指针对应的副本，由于是局部变量，所以调用结束后会被销毁，所以并没有任何内存问题。
+
+# C++
+
+## 思路一
+``` C++
+class PeekingIterator : public Iterator {
+private:
+    bool flag;
+    int value;
+    
+public:
+    PeekingIterator(const vector<int>& nums) : Iterator(nums) {
+	flag = false; // value是否存放了peek和next应该返回的值
+    }
+	
+    int peek(){
+        if(!flag) value = Iterator::next();
+        flag = true;
+        return value;
+    }
+
+    int next() {
+        if(!flag) return Iterator::next();
+        flag = false;
+        return value;
+    }
+
+    bool hasNext() const {
+        return flag || Iterator::hasNext();
+    }
+};
+```
+
+## 思路二
+``` C++
+class PeekingIterator : public Iterator {
+public:
+    PeekingIterator(const vector<int>& nums) : Iterator(nums) {}
+	
+    int peek(){
+        // 调用Iterator(const Iterator& iter)创造一个副本
+        return Iterator(*this).next();
+    }
+
+//     int next() {} 
+//     bool hasNext() const {}
+};
+```
+