Sort Colors

You are given an integer array nums containing only 0, 1, and 2. Rearrange the array in place so that equal values are adjacent and the values appear in the order 0, 1, then 2.
Do not call a library sorting routine. Return the same array after rearranging it so the result can be evaluated.

Function
sortColors(nums: int[]) → int[]

Examples
Example 1
nums = [2,0,2,1,1,0]
return = [0,0,1,1,2,2]
The two zeros come first, followed by the two ones and the two twos.
Example 2
nums = [2,0,1]
return = [0,1,2]
Each color occurs once.

Constraints
1 <= nums.length <= 100000
nums[i] is 0, 1, or 2.
The rearrangement must use constant auxiliary space.

The trick is the Dutch National Flag partition. Keep three pointers: low, mid and high. Low marks where the next 0 goes, high marks where the next 2 goes, and mid scans. If nums[mid] is 0, swap with low and advance both. If it's 1, just advance mid. If it's 2, swap with high and shrink high, but do NOT advance mid, because the swapped-in value is unchecked. That last point is the classic pitfall, and it breaks the output on inputs like [2,0,1]. The counting approach (tally 0s, 1s, 2s, then overwrite) also meets constant space and is fine, but it takes two passes, and an interviewer may push for one. Runtime is O(n) and space is O(1). With n up to 100000 nothing else matters. If your mind goes blank mid-assessment, StealthCoder is the hedge that shows the pointer swaps on screen without the proctor seeing it.

Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.