Summary Ranges

Problem

https://leetcode.com/problems/summary-ranges/

You are given a sorted unique integer array nums.

A range [a,b] is the set of all integers from a to b (inclusive).

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

• -2:sup:`31`<= nums[i] <= 2:sup:`31`- 1

• All the values of nums are unique.

• nums is sorted in ascending order.

Pattern

Array

Solution

Note that subarray of consecutive numbers is a range. Iterate through the array and keep track of the start of the range. Keep iterating as long as the next element is consecutive. When the next element is not consecutive, create the range string based on whether the start equals the end.

Code

from typing import List


def summaryRanges(nums: List[int]) -> List[str]:
    """Return the smallest sorted list of ranges that cover all the numbers in
    ``nums``.
    """
    summary_ranges = []

    i = 0
    while i < len(nums):
        start = nums[i]

        while i + 1 < len(nums) and nums[i] + 1 == nums[i + 1]:
            i += 1

        summary_ranges.append(get_range_str(start, nums[i]))
        i += 1

    return summary_ranges


def get_range_str(start, end):
    """Get the range from ``start`` to ``end`` as a formatted string.
    """
    return str(start) if start == end else f'{start}->{end}'

Test

>>> from summary_ranges__approach_1 import summaryRanges
>>> summaryRanges([0, 1, 2, 4, 5, 7])
['0->2', '4->5', '7']
>>> summaryRanges([0, 2, 3, 4, 6, 8, 9])
['0', '2->4', '6', '8->9']

Complexity

\(n\) is the length of the input array

Time: \(O(n)\) — single pass through the array

Auxiliary Space: \(O(1)\)

summary_ranges__approach_1.summaryRanges(nums: List[int]) → List[str]

Return the smallest sorted list of ranges that cover all the numbers in nums.

summary_ranges__approach_1.get_range_str(start, end)

Get the range from start to end as a formatted string.