Clarify that the array contains only 0, 1, and 2, and that sorting must be in-place. Propose the Dutch National Flag algorithm using three pointers (low, mid, high) to partition the array into three regions in a single pass. Walk through the pointer movement and swapping logic, emphasizing O(n) time and O(1) space.
Pro tip: Mention that this is a variation of the Dutch National Flag problem and that the same partitioning logic can be extended to three-way quicksort. Also, discuss edge cases like empty arrays or arrays with only one distinct value to show thoroughness.
Confirm that the array contains only 0s, 1s, and 2s, and that sorting should be done in-place with O(1) extra space. Ask if the array can be modified and if stability matters.
Select the Dutch National Flag algorithm (three-pointer approach) because it sorts in a single pass with constant space. Explain that it partitions the array into three sections: 0s, 1s, and 2s.
Set low = 0, mid = 0, and high = n-1. These pointers define the boundaries: elements before low are 0s, between low and mid are 1s, and after high are 2s.
While mid <= high, inspect arr[mid]. If it's 0, swap with arr[low] and increment both low and mid. If it's 1, just increment mid. If it's 2, swap with arr[high] and decrement high (without incrementing mid).
State that time complexity is O(n) and space is O(1). Discuss edge cases: empty array, all elements same, or already sorted. Mention that the algorithm is not stable but stability isn't required here.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.