Problem of The Day: Happy Number

Problem Statement

Intuition

The idea is likely to iterate through the list and identify consecutive ranges of numbers.

Approach

My approach is to iterate through the list of numbers while keeping track of the start and end of each range. If the current number is not consecutive to the previous one, I’ll add the range to the result list. I’ll handle the case where a range has only one element separately.

Complexity

• Time complexity: O(n)

• Space complexity: O(n)

Code

class Solution:
    def summaryRanges(self, nums: List[int]) -> List[str]:
        if not nums:
            return []
        res = []
        N = len(nums)
        start = 0
        end = 1
        for end in range(1, N):
            if nums[end] != nums[end - 1] + 1:
                if start != end - 1:
                    res.append(str(nums[start]) + "->" + str(nums[end - 1]))
                else:
                    res.append(str(nums[start]))
                start = end
        
        if start != end and end < len(nums):
            res.append(str(nums[start]) + "->" + str(nums[end]))
        else:
            res.append(str(nums[start]))
        return res

Editorial Solution

Approach: Fix Left Bound

class Solution:
    def summaryRanges(self, nums: List[int]) -> List[str]:
        ranges = []     
        i = 0 
        
        while i < len(nums): 
            start = nums[i]  
            while i + 1 < len(nums) and nums[i] + 1 == nums[i + 1]: 
                i += 1 
            
            if start != nums[i]: 
                ranges.append(str(start) + "->" + str(nums[i]))
            else: 
                ranges.append(str(nums[i]))
            
            i += 1

        return ranges

• Time complexity: O(n)
• Space complexity: O(n)