sort | uniq -c | sort -n

sortuniq.cpp

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>

typedef std::map<const std::string, unsigned> map_t;
typedef std::pair<unsigned, const std::string*> pair_t;
typedef std::vector<pair_t> array_t;

static bool comparepair(const pair_t &a, const pair_t &b) {
  return a.first > b.first;
}

int main() {
  map_t map;
  // Gather line counts.
  while (std::cin.good()) {
    std::string line;
    std::getline(std::cin, line);
    ++map[line];
  }

  // Make a copy to sort.
  array_t array;
  array.reserve(map.size());
  for (map_t::const_iterator i = map.begin(), e = map.end(); i != e; ++i)
    array.push_back(pair_t(i->second, &i->first));

  std::sort(array.begin(), array.end(), comparepair);

  for (array_t::const_iterator i = array.begin(), e = array.end(); i != e; ++i)
    std::cout << i->first << '\t' << *i->second << std::endl;
}