Summary Ranges - Solution & Explanation

Problem Overview: You receive a sorted array of unique integers. The task is to compress consecutive values into readable ranges. For example, [0,1,2,4,5,7] becomes ["0->2","4->5","7"]. Each continuous sequence should be represented by its start and end.

Approach 1: Sequential Range Detection (Time: O(n), Space: O(1) excluding output)

This method scans the array from left to right and builds ranges as soon as a break in consecutiveness appears. Start by marking the first element as the beginning of a range. Continue iterating while the next value equals current + 1. When the sequence breaks, close the range and append it to the result. If the start and end are the same, output a single number; otherwise format it as start->end. The algorithm performs one linear pass, so the time complexity stays O(n) with constant auxiliary space.

This solution relies only on simple iteration over an array. It's easy to implement and works well because the input is already sorted and contains unique values. Every number is visited exactly once.

Approach 2: Optimal Pointer Movement (Time: O(n), Space: O(1) excluding output)

This approach uses a two-pointer scanning pattern. Maintain two indices: start marking the beginning of a range and i expanding forward. While nums[i] == nums[i-1] + 1, keep extending the range. Once the condition fails, finalize the range from start to i-1 and reset start = i. The two pointers effectively track the current window of consecutive values.

The logic is identical to detecting continuous segments in many array traversal problems. The technique resembles a sliding window without shrink operations, making it a common pattern in two pointers problems. Each element advances the pointer once, so the runtime remains O(n) and no additional memory structures are required.

The advantage of the pointer-based version is clarity in interviews. Interviewers often expect candidates to identify consecutive segments using pointer movement instead of nested loops. It also scales naturally to similar problems such as merging intervals or detecting streaks.

Recommended for interviews: Start by explaining the sequential detection idea since it directly models the problem statement. Then implement the pointer-based scan, which interviewers usually view as the cleanest solution. Both run in O(n) time, but the pointer formulation demonstrates stronger control over iteration patterns and familiarity with two-pointer techniques.