Sorting — Sort Colors (Dutch National Flag)

solve(nums) — nums only contains 0, 1 and 2. Sort it in place so the
zeros come first, then the ones, then the twos, and return it. Do it in one pass
with constant extra space — no sort(), no counting pass.

Keep three regions with two boundaries and one scanner: everything before low is
0, everything after high is 2, and the scanner mid walks the unknown middle.

Examples

solve([2,0,2,1,1,0])  → [0,0,1,1,2,2]
solve([2,0,1])        → [0,1,2]
solve([1,1,1])        → [1,1,1]