> For the complete documentation index, see [llms.txt](https://code.taozirui.com/lc/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://code.taozirui.com/lc/math/reverse-integer.md).

# 7. Reverse Integer

Given a 32-bit signed integer, reverse digits of an integer.

**Example 1:**

```
Input: 123

Output: 321
```

**Example 2:**

```
Input: -123

Output: -321
```

**Example 3:**

```
Input: 120

Output: 21
```

**Note:**\
**Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range**: \[−231, 231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

**Thoughts:**

1. Use String: 
   1. sign: 
   2. reverse abs value: sign \* x \[::-1]
   3. s \* r\* (r < 2 ^31)
2. Not use string: Standard way of detecting overflow

```python
class Solution(object):
    def reverse(self, x):
        """
        :type x: int
        :rtype: int
        """
        #sign 
        s = cmp(x,0)
        # reverse abs
        r = int(`s*x`[::-1])
        return s * r * (r < 2 ** 31)
```

```java
class Solution {
    public int reverse(int x){
        int result = 0;

        while (x != 0)
        {
            int tail = x % 10;
            int newResult = result * 10 + tail;
            if ((newResult - tail) / 10 != result) return 0;
            result = newResult;
            x /= 10;
        }
        return result;
    }
}
```