Sort Colors - Three Pointers

The Dutch National Flag algorithm uses $3$ pointers: low, mid, and high. The idea is to partition the array into three regions as you scan.

$1.$ Everything before low is $0$ (red).

$2.$ Everything between low and mid is $1$ (white).

$3.$ Everything after high is $2$ (blue).

$4.$ Everything between mid and high is unprocessed.

You process elements at mid and swap them into the correct region.