You are given an inclusive range [lower, upper] and a sorted unique integer array nums, where all elements are in the inclusive range.

A number x is considered missing if x is in the range [lower, upper] and x is not in nums.

Return the smallest sorted list of ranges that cover every missing number exactly. That is, no element of nums is in any of the ranges, and each missing number is in one of the ranges.

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

• "a->b" if a != b
• "a" if a == b

 

Example 1:

Input: nums = [0,1,3,50,75], lower = 0, upper = 99
Output: ["2","4->49","51->74","76->99"]
Explanation: The ranges are:
[2,2] --> "2"
[4,49] --> "4->49"
[51,74] --> "51->74"
[76,99] --> "76->99"

Example 2:

Input: nums = [], lower = 1, upper = 1
Output: ["1"]
Explanation: The only missing range is [1,1], which becomes "1".

Example 3:

Input: nums = [], lower = -3, upper = -1
Output: ["-3->-1"]
Explanation: The only missing range is [-3,-1], which becomes "-3->-1".

Example 4:

Input: nums = [-1], lower = -1, upper = -1
Output: []
Explanation: There are no missing ranges since there are no missing numbers.

Example 5:

Input: nums = [-1], lower = -2, upper = -1
Output: ["-2"]

 

Constraints:

• -109 <= lower <= upper <= 109
• 0 <= nums.length <= 100
• lower <= nums[i] <= upper
• All the values of nums are unique.

 

 

Ideas:  t: O(n)

1. 建一个helper function来得到range

2. for loop, check num 和lower的大小，如果相等，lower += 1； 如果lower 比较小，将range [lower, num - 1] append 进ans，同时lower += 1

3. 注意finalRange， 需要将finalRange加入到ans中

 

Code:

class Solution:
    def findMissingRanges(self, nums: List[int], lower: int, upper: int) -> List[str]:
        n, ans = len(nums), []
        for num in nums:
            if num == lower:
                lower += 1
            elif lower < num:
                ans.append(self.generateRange(lower, num - 1))
                lower = num + 1
        finalRange = self.generateRange(lower, upper)
        if finalRange:
            ans.append(finalRange)
        return ans
    
    def generateRange(self, lower, upper) -> str:
        if lower == upper:
            return str(lower)
        elif lower < upper:
            return str(lower) + '->' + str(upper)
        return ""