Dutch National Flag – Algorithm + TDD

The algorithm is pretty simple and achievable in O(n) time where n is the number of elements. Instead of using indexes, I am using 3 set of iterators and added bunch of test cases.

```#include <gtest/gtest.h>
#include <algorithm> // random_shuffle
#include <vector>
#include <numeric> // accumulate

using it = std::vector<int>::iterator; // using typedef

void dnf(it less, it equal, it greater, int pivot){
while(equal <= greater){
if(*equal < pivot){
std::iter_swap(less, equal);
less++;
equal++;
}else if(*equal == pivot){
equal++;
}else{
std::iter_swap(equal, greater);
greater--;
}
}
}

TEST(RandomTest, dnfTest){
std::vector<int> v{2, 2, 0, 1, 1, 0, 2, 1, 0, 1, 0, 1, 0, 2, 2};
int pivot = 1;
for(int i = 0; i < 1000; i++){
dnf(v.begin(), v.begin(), v.end()-1, 1);
std::string output = std::accumulate(std::next(v.begin()), v.end(),
std::to_string(*v.begin()),
[](std::string s, int i){return s+= std::to_string(i);});

EXPECT_EQ(output, "000001111122222");
}
}

TEST(RandomTestTwoNumbers, dnfTest){
std::vector<int> v{0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1, 1};
int pivot = 1;
for(int i = 0; i < 1000; i++){
dnf(v.begin(), v.begin(), v.end()-1, 1);
std::string output = std::accumulate(std::next(v.begin()), v.end(),
std::to_string(*v.begin()),
[](std::string s, int i){return s+= std::to_string(i);});

EXPECT_EQ(output, "000000001111111");
}
}

TEST(RandomTestTwoNumbersMidPivot, dnfTest){
std::vector<int> v{0, 0, 0, 0,0,9,9,9,9,9,9,9,9};
int pivot = 1;
for(int i = 0; i < 1000; i++){
dnf(v.begin(), v.begin(), v.end()-1, 1);
std::string output = std::accumulate(std::next(v.begin()), v.end(),
std::to_string(*v.begin()),
[](std::string s, int i){return s+= std::to_string(i);});

EXPECT_EQ(output, "0000099999999");
}
}

TEST(RandomTestTwoNumbersHighPivot, dnfTest){
std::vector<int> v{0, 0, 0, 0,0,9,9,9,9,9,9,9,9};
int pivot = 10;
for(int i = 0; i < 1000; i++){
dnf(v.begin(), v.begin(), v.end()-1, 1);
std::string output = std::accumulate(std::next(v.begin()), v.end(),
std::to_string(*v.begin()),
[](std::string s, int i){return s+= std::to_string(i);});

EXPECT_EQ(output, "0000099999999");
}
}

int main(int argc, char**argv, char**envArg)
{