#include <iostream>
#include <algorithm>
#include <string>
#include <set>

int number(const std::string& key, const std::string& permutation) {
  int result = 0;
  for ( int i = 0; i < key.size(); ++i ) {
    result = result * 10 + permutation.find(key[i]);
  }
  return result;
}

std::string digits(const std::string& key, const std::string& permutation) {
  std::string result;
  for ( int i = 0; i < key.size(); ++i ) {
    result += (permutation.find(key[i]) + '0');
  }
  return result;
}

int main() {
  const std::string A = "SEND";
  const std::string B = "MORE";
  const std::string C = "MONEY";
  std::set<char> pset;
  pset.insert(A.begin(), A.end());
  pset.insert(B.begin(), B.end());
  pset.insert(C.begin(), C.end());
  if ( pset.size() > 10 ) {
    std::cerr << "Too many digits!" << std::endl;
    return -1;
  }
  std::string unique(pset.begin(), pset.end());
  std::string permutation = unique;
  std::set<std::string> answers;
  while ( permutation.size() != 10 ) permutation += ' ';
  do {
    int na = number(A, permutation);
    int nb = number(B, permutation);
    int nc = number(C, permutation);
    if ( na + nb == nc ) {
      if ( answers.insert(digits(unique, permutation)).second ) {
        std::cout << digits(A, permutation) << " + "
                  << digits(B, permutation) << " = "
                  << digits(C, permutation) << std::endl;
      }
    }
  } while ( next_permutation(permutation.begin(), permutation.end()) );
  return 0;
}

#include <iostream>
#include <algorithm>
#include <string>
#include <set>
#include <cassert>

class permutation {
private:
  std::string permutation_;
  std::string unique_;
public:
  typedef std::pair<int,std::string> pair_type;
  explicit permutation(const std::string& keywords) {
    std::set<char> unique(keywords.begin(), keywords.end());
    assert( unique.size() <= 10 );
    permutation_.assign(unique.begin(),unique.end());
    unique_ = permutation_;
    while ( permutation_.size() != 10 ) {
      permutation_ += ' ';
    }
  }
  pair_type pair(const std::string& key) const {
    pair_type result(0,"");
    for ( int i = 0; i < key.size(); ++i ) {
      int n = permutation_.find(key[i]);
      result.first = result.first * 10 + n;
      result.second += (n + '0');
    }
    return result;
  }
  std::string hash() const {
    return pair(unique_).second;
  }
  bool next() {
    return next_permutation(permutation_.begin(), permutation_.end());
  }
};

int main() {
  const std::string A = "SEND";
  const std::string B = "MORE";
  const std::string C = "MONEY";

  permutation perm(A+B+C);
  std::set<std::string> answers;
  do {
    permutation::pair_type na = perm.pair(A);
    permutation::pair_type nb = perm.pair(B);
    permutation::pair_type nc = perm.pair(C);
    if ( na.first + nb.first == nc.first ) {
      if ( answers.insert(perm.hash()).second ) {
        std::cout << na.second << " + "
                  << nb.second << " = "
                  << nc.second << std::endl;
      }
    }
  } while ( perm.next() );
  return 0;
}

#include <stdio.h>
#include <string.h>

int perm(char *s, int n)
{
    int i = n, j, k, t;
    while (--i > 0 && s[i-1] >= s[i]) ;
    if (i <= 0) return 0;
    for (k = i, j = n; i < --j; i++) t = s[i], s[i] = s[j], s[j] = t;
    for (t = s[k-1], i = k; s[i] <= t; i++) ;
    s[k-1] = s[i], s[i] = t;
    return 1;
}

int val(const char *s, const char *p)
{
    int n = 0;
    while (*p) n = n * 10 + strchr(s, *p++) - s;
    return n;
}

int main(void)
{
    char s[] = "DEMNORSY--";
    do {
        int a = val(s, "SEND"), b =val(s, "MORE"), c =val(s, "MONEY");
        if (a + b == c) printf("%04d + %04d = %05d\n", a, b, c);
    } while (perm(s, 10));
    return 0;
}

>   while (*p) n = n * 10 + strchr(s, *p++) - s;

訂正
    while (*p) n = n * 10 + (strchr(s, *p++) - s);

>   char s[] = "DEMNORSY--";

訂正
    char s[] = "--DEMNORSY";

こうしないと、A + B = C が解けません。

επιστημηさんのも、' ' を前につけるか、あるいは
後ろにつけたいのなら 'Z' より大きいものにしないとだめでしょう。

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

int perm(char *s, int n)
{
    int i = n, j, k, t;
    while (--i > 0 && s[i-1] >= s[i]) ;
    for (k = i, j = n; i < --j; i++) t = s[i], s[i] = s[j], s[j] = t;
    if (k <= 0) return 0;
    for (t = s[k-1], i = k; s[i] <= t; i++) ;
    s[k-1] = s[i], s[i] = t;
    return 1;
}

int val(const char *s, const char *p)
{
    int n = 0;
    while (*p) n = n * 10 + (strchr(s, *p++) - s);
    return n;
}

int append(char *s, int n, const char *p)
{
    for (; *p; p++)
        if (!strchr(s, *p)) {
            if (n >= 10) exit(1);
            s[n++] = *p;
        }
    return n;
}

int comp(const void *a, const void *b)
{
    return *(char *)a - *(char *)b;
}

int main(void)
{
    char x[256], y[256], z[256], s[11] = "";  int n, a, b, c;

    if (!gets(x) || !gets(y) || !gets(z)) return 1;
    n = append(s, 0, x), n = append(s, n, y), n = append(s, n, z);
    while (n < 10) s[n++] = ' ';
    qsort(s, 10, 1, comp);
    do {
        a = val(s, x), b = val(s, y), c = val(s, z);
        if (a + b == c && *x != *s && *y != *s && *z != *s)
            printf("%d + %d = %d\n", a, b, c);
    } while (perm(s, 10));
    return 0;
}