Determine if one string is a permutation of another

is_permutation.cc

#include <string>
#include <map>

bool IsPermutation(std::string &a, std::string &b) {
  std::map<char, std::pair<int, int>> char_map;
  for (auto c : a) ++char_map[c].first;
  for (auto c : b) ++char_map[c].second;
  for (auto it : char_map)
    if (it.second.first != it.second.second) return false;
  return true;
}