# 228. Summary Ranges

#### Easy

***

You are given a **sorted unique** integer array `nums`.

Return *the **smallest sorted** list of ranges that **cover all the numbers in the array exactly***. That is, each element of `nums` is covered by exactly one of the ranges, and there is no integer `x` such that `x` is in one of the ranges but not in `nums`.

Each range `[a,b]` in the list should be output as:

* `"a->b"` if `a != b`
* `"a"` if `a == b`

 

**Example 1:**

```
Input: nums = [0,1,2,4,5,7]
Output: ["0->2","4->5","7"]
Explanation: The ranges are:
[0,2] --> "0->2"
[4,5] --> "4->5"
[7,7] --> "7"
```

**Example 2:**

```
Input: nums = [0,2,3,4,6,8,9]
Output: ["0","2->4","6","8->9"]
Explanation: The ranges are:
[0,0] --> "0"
[2,4] --> "2->4"
[6,6] --> "6"
[8,9] --> "8->9"
```

 

**Constraints:**

* `0 <= nums.length <= 20`
* `-231 <= nums[i] <= 231 - 1`
* All the values of `nums` are **unique**.
* `nums` is sorted in ascending order.

```python
class Solution:
    def summaryRanges(self, nums: List[int]) -> List[str]:
        if not nums:
            return []
        left = right = None
        result = []
        for num in nums:
            if left is None:
                left = right = num
                continue
            if (num-right) == 1:
                right = num
            elif num - left > 1:
                if left == right:
                    result.append(f"{left}")
                else:
                    result.append(f"{left}->{right}")
                left = right = num
        if left == right:
            result.append(f"{left}")
        else:
            result.append(f"{left}->{right}")
        return result
```