A real coding interview, start to finish

What happens in the video

Sarah, the interviewer, runs a two-part technical round over a video call and a shared coding pad. Jake, the candidate, has CoPilot Interview open next to the pad. Each step below links to the moment in the video.

Coding question: Merge Intervals

Given a list of intervals [start, end], merge every overlapping pair and return the intervals that remain. For example, [[1,3],[2,6],[8,10],[15,18]] becomes [[1,6],[8,10],[15,18]], and intervals that touch, like [1,4] and [4,5], merge too.

The approach the app gives and Jake explains: sort the intervals by their start, then walk through them once. If the next interval starts before the last merged one ends, extend that end; otherwise, add it as a new interval.

def merge(intervals):
    if not intervals:
        return []

    intervals.sort(key=lambda x: x[0])
    merged = [list(intervals[0])]

    for current in intervals[1:]:
        last = merged[-1]
        if current[0] <= last[1]:
            last[1] = max(last[1], current[1])
        else:
            merged.append(list(current))

    return merged

Complexity: O(n log n) time for the sort and O(n) space for the output.

Debugging question: a binary search that never finishes

binary_search(nums, target) should return the index of target in a sorted list, or -1. On [1, 3, 5, 7, 9] with target 9, the loop runs forever.

def binary_search(nums, target):
    left, right = 0, len(nums) - 1
    while left <= right:
        mid = (left + right) // 2
        if nums[mid] == target:
            return mid
        elif nums[mid] < target:
            left = mid        # line 8
        else:
            right = mid       # line 10
    return -1

With Debug code on, the app answers with a bug table:

Once mid has been checked it must leave the search range. With both bounds moving past it, every pass makes progress, and the two tests return 4 and -1.

How CoPilot Interview helps in a coding round

More detail: coding interview help, the coding interview cheat sheet, and AI mock interviews to rehearse a round first.