Sort an array of 0s, 1s and 2s

Given an array A[] consisting 0s, 1s and 2s, write a function that sorts A[]. The functions should put all 0s first, then all 1s and all 2s in last.

Example
Input = {0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1};
Output = {0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2}

This is a variation of famous Dutch national flag problem
The problem was posed with three colours, here `0?, `1? and `2?. The array is divided into four sections:

a[1..Lo-1] zeroes (red)
a[Lo..Mid-] ones (white)
a[Mid..Hi] unknown
a[Hi+1..N] twos (blue)
The unknown region is shrunk while maintaining these conditions

Lo := 1; Mid := 1; Hi := N;
while Mid <= Hi do
Invariant: a[1..Lo-1]=0 and a[Lo..Mid-1]=1 and 
a[Hi+1..N]=2; a[Mid..Hi] are unknown.

case a[Mid] in
0: swap a[Lo] and a[Mid]; Lo++; Mid++
1: Mid++
2: swap a[Mid] and a[Hi]; Hi–

http://www.geeksforgeeks.org/sort-an-array-of-0s-1s-and-2s/