#include <stdio.h>
#define N 5
#define M 2

void display(char* stack, int size)
{
        printf("%s\n", stack);
}

void recurse(int n, int m, char* stack, int filled = 0)
{
        // filled denotes how many elements of our stack are filled [ with valid data ]
        if (filled == 0)
        {
                // initiallize our base cases, 0 though (n - m)
                //  - no point in setting anything higher than n-m because it will be pruned
                for (int x = 0; x <= n - m; ++x)
                {
                        stack[0] = 'a' + x;     // 'b' = 'a' + 1, 'c' = 'a' + 2, etc.
                        recurse(n, m, stack, filled + 1);
                }
                return;
        }
        if (filled == m)
        {
                // if our stack is full, display it
                display(stack, m);
                return;
        }
        // stack[filled - 1] was the last char we put in.
        //  extract that number (as element of n) by subtracting 'a'
        // starting at the next element, iterate up as far to n as will still allow us to fill
        // any unfilled elements of the stack (m - filled), thus, n - (m - filled)
        for (int x = (int)(stack[filled - 1] - 'a') + 1; x <= n - (m - filled); ++x)
        {
                stack[filled] = 'a' + x;
                recurse(n, m, stack, filled + 1);
        }
        return;
}

int main()
{
        char* stack = new char[M + 1];
        stack[M] = '\0';
        recurse(N, M, stack);
        delete[] stack;
}