My solution for https://www.research.ibm.com/haifa/ponderthis/challenges/May2017.html

a.c

#include <stdio.h>
#include <stdlib.h>

#define N 26
#define LEN (N * 2 + 1)

static int s[LEN] ;

static void print_s(void) {
    for (int i = 0; i < LEN; i ++)
        printf("%c", s[i] == 1 ? '-' : (s[i] - 3 + 'A')) ;
    puts("") ;
    exit(0) ;
}

static int add_char(char c, unsigned i) {
    int i2 = i + c - 1 ;
    if (s[i] == 0 && s[i2] == 0) {
        s[i] = s[i2] = c ;
        return 0 ;
    }
    return -1 ;
}

static void del_char(char c, unsigned i) {
    s[i] = s[i + c - 1] = 0 ;
}

#define X(n, Q)    \
    for (int i##n = !n; i##n < LEN - n; i##n ++) \
        if (add_char(n + 1, i##n) == 0) { \
            Q ; \
            del_char(n + 1, i##n) ; \
        }

int main(int ac, const char *av[]) {
    X(0, X(27, X(26, X(25, X(24, X(23, X(22, X(21, X(20, X(19, X(18, X(17, X(16, X(15, X(14, X(13, X(12, X(11, X(10, X(9, X(8, X(7, X(6, X(5, X(4, X(3, X(2, print_s()))))))))))))))))))))))))))) ;
    return 0 ;
}