Update solution 007

Author: yanglbme

README.md

@@ -13,6 +13,7 @@ Complete solutions to Leetcode problems, updated daily.
 | # | Title | Tags |
 |---|---|---|
 | 001 | [Two Sum](https://github.com/yanglbme/leetcode/tree/master/solution/001.Two%20Sum) | `Array`, `Hash Table` |
+| 007 | [Reverse Integer](https://github.com/yanglbme/leetcode/tree/master/solution/007.Reverse%20Integer) | `Math` |
 | 021 | [Merge Two Sorted Lists](https://github.com/yanglbme/leetcode/tree/master/solution/021.Merge%20Two%20Sorted%20Lists) | `Linked List` |
 | 083 | [Remove Duplicates from Sorted List](https://github.com/yanglbme/leetcode/tree/master/solution/083.Remove%20Duplicates%20from%20Sorted%20List) | `Linked List` |
 | 203 | [Remove Linked List Elements](https://github.com/yanglbme/leetcode/tree/master/solution/203.Remove%20Linked%20List%20Elements) | `Linked List` |

solution/007.Reverse Integer/README.md

@@ -0,0 +1,51 @@
+## 反转整数
+### 题目描述
+
+给定一个 32 位有符号整数，将整数中的数字进行反转。
+
+示例 1:
+```
+输入: 123
+输出: 321
+```
+
+ 示例 2:
+```
+输入: -123
+输出: -321
+```
+
+示例 3:
+```
+输入: 120
+输出: 21
+```
+
+注意:
+
+假设我们的环境只能存储 32 位有符号整数，其数值范围是 [−231,  231 − 1]。根据这个假设，如果反转后的整数溢出，则返回 0。
+
+### 解法
+用 long 型存储该整数，取绝对值，然后转成 StringBuilder 进行 reverse，后转回 int。注意判断该数是否在[Integer.MIN_VALUE, Intger.MAX_VALUE] 范围内。
+
+```java
+class Solution {
+    public int reverse(int x) {
+        if (x == 0) {
+            return x;
+        }
+        
+        long tmp = x;
+        boolean isPositive = true;
+        if (tmp < 0) {
+            isPositive = false;
+            tmp = -tmp;
+        }
+        
+        long val = Long.parseLong(new StringBuilder(String.valueOf(tmp)).reverse().toString());
+        
+        return isPositive ? (val > Integer.MAX_VALUE ? 0 : (int) val) : (-val < Integer.MIN_VALUE ? 0 : (int) (-val)); 
+        
+    }
+}
+```

solution/007.Reverse Integer/Solution.java

@@ -0,0 +1,19 @@
+class Solution {
+    public int reverse(int x) {
+        if (x == 0) {
+            return x;
+        }
+        
+        long tmp = x;
+        boolean isPositive = true;
+        if (tmp < 0) {
+            isPositive = false;
+            tmp = -tmp;
+        }
+        
+        long val = Long.parseLong(new StringBuilder(String.valueOf(tmp)).reverse().toString());
+        
+        return isPositive ? (val > Integer.MAX_VALUE ? 0 : (int) val) : (-val < Integer.MIN_VALUE ? 0 : (int) (-val)); 
+        
+    }
+}