试题:组合的模板非递归实现

#include <cassert>
#include <vector>
#include <iostream>
using namespace std;

template <typename T>
void show(const vector<T> & element, const vector<int> & index)
{
assert(index.size() > 0);
assert(element.size() >= index.size());
for (int i = 0; i < index.size(); ++i) {
cout << element[index[i]] << " ";
}
cout << endl;
}

template <typename T>
void select(const vector<T> & element, int n, vector<int> & index)
{
if (n <= 0 || n > element.size()) {
return;
}

index.clear();
for (int i = 0; i < n; ++i) {
index.push_back(i);
}

while (true) {
while (index[n - 1] <= element.size() - 1) {
show(element, index);
++index[n - 1];
}

int index_bound = element.size();
int carry_index = n - 2;
while (carry_index >= 0) {
++index[carry_index];
--index_bound;
if (index[carry_index] < index_bound) {
break;
}
else {
assert(index[carry_index] == index_bound);
--carry_index;
}
}
if (carry_index >= 0) {
for (int i = carry_index + 1; i < n; ++i) {
index[i] = index[i - 1] + 1;
}
}
else {
break;
}
}
}

template <typename U, typename V>
void combine(U begin, U end, V * pfirst)
{
vector<V> element(begin, end);
vector<int> index;
index.reserve(element.size());

for (int i = 1; i <= element.size(); ++i) {
select(element, i, index);
}
}

template <typename T>
void combine(T begin, T end)
{
combine(begin, end, &*begin);
}

int main()
{
int data[] = { 1, 2, 3, 4, 5, 6 };
combine(data, data + sizeof(data) / sizeof(data[0]));
combine(data, *(&data + 1));

return 0;
}