C# / .NET Mid-level (2–5 yrs) Coding Very frequent C# intervals sorting LeetCode arrays

Merge overlapping intervals from a list of [start, end] pairs

cs-mid-006

Your answer

Answer as you would in a real interview — explain your thinking, not just the conclusion.

Model answer

Sort the intervals by start time. Iterate and compare each interval to the last element in the result list. If the current start is less than or equal to the last result's end, they overlap — extend the end if needed. Otherwise append the current interval as a new entry. This is O(n log n) for the sort and O(n) for the merge pass.

Code example

public int[][] Merge(int[][] intervals)
{
    if (intervals.Length <= 1) return intervals;

    // Sort by start
    Array.Sort(intervals, (a, b) => a[0].CompareTo(b[0]));

    var result = new List<int[]> { intervals[0] };

    for (int i = 1; i < intervals.Length; i++)
    {
        var last = result[^1];
        if (intervals[i][0] <= last[1])          // overlapping
            last[1] = Math.Max(last[1], intervals[i][1]);
        else
            result.Add(intervals[i]);
    }

    return result.ToArray();
}

// Test
var merged = Merge(new[] { new[]{1,3}, new[]{2,6}, new[]{8,10}, new[]{15,18} });
// [[1,6],[8,10],[15,18]]

Follow-up

How would you handle this if intervals arrive in a stream and you need to maintain a sorted set?