Simple implementation based on the O(n) counting method from the problem's follow-up, scores within the 90th percentile of all C# submissions.

```
public class Solution {
private const int RED = 0;
private const int WHITE = 1;
private const int BLUE = 2;
public void SortColors(int[] nums) {
int redCount = 0, whiteCount = 0, blueCount = 0;
for(int i = 0; i < nums.Length; i++) {
if(nums[i] == RED) {
redCount++;
}
else if(nums[i] == WHITE) {
whiteCount++;
}
else if(nums[i] == BLUE) {
blueCount++;
}
}
int iter = 0;
for(int i = 0; i < redCount; i++) {
nums[iter] = RED;
iter++;
}
for(int i = 0; i < whiteCount; i++) {
nums[iter] = WHITE;
iter++;
}
for(int i = 0; i < blueCount; i++) {
nums[iter] = BLUE;
iter++;
}
}
}
```