c/c++语言开发共享C++代码实现逆波兰式

  /// file: reversepolishnotation.h  #include <string>  #include <stack>    class reversepolishnotation {  private:   std::string _expr;   unsigned _idx;   std::stack<std::string> _stk;  public:   reversepolishnotation(const std::string &expr);     std::string nextword();     std::string operator()();     static int getoppriority(const std::string &word);     bool isword(const std::string &word);     bool isoperator(const std::string &word);  };

  /// file: reversepolishnotation.cpp  #include <iostream>  #include <cassert>  #include "reversepolishnotation.h"  #include <cctype>  #include <sstream>    using std::cout;  using std::endl;    reversepolishnotation::reversepolishnotation(   const std::string &expr) : _idx(0), _expr(expr) {}    std::string reversepolishnotation::nextword() {   if (_idx >= _expr.length()) {   return "";   }   return _expr.substr(_idx++, 1);  }    std::string reversepolishnotation::operator()() {   std::stringstream outexpr;   std::string word;   std::string topelem;   while (true) {   word = nextword();   if (isword(word)) {   outexpr << word << " ";   } else if (isoperator(word)) {   if (_stk.empty() || _stk.top() == "(") {   _stk.push(word);   continue;   }   topelem = _stk.top();   while (getoppriority(topelem) > getoppriority(word)) {   outexpr << topelem << " ";   _stk.pop();   if (_stk.empty()) {    break;   }   topelem = _stk.top();   }   _stk.push(word);     } else if (word == "(") {   _stk.push(word);   } else if (word == ")") {   while (true) {   topelem = _stk.top();   _stk.pop();   if (topelem == "(") {    break;   }   assert(!_stk.empty() && "[e]expr error. missing '('.");   outexpr << topelem << " ";   }   } else if (word == "") {   while (!_stk.empty()) {   topelem = _stk.top();   assert (topelem != "(" && "[e]expr error. redundant '('.");   outexpr << topelem << " ";   _stk.pop();   }   break;   } else {   assert(false && "[w]>>>can not recognize this word");   }   }   return outexpr.str();  }    int reversepolishnotation::getoppriority(const std::string &word) {   if (word == "+") { return 1; }   if (word == "-") { return 1; }   if (word == "*") { return 2; }   if (word == "/") { return 2; }   return 0;  }    bool reversepolishnotation::isword(const std::string &word) {   return isalpha(word.c_str()[0]) || isdigit(word.c_str()[0]);  }    bool reversepolishnotation::isoperator(const std::string &word) {   return word == "+" ||    word == "-" ||    word == "*" ||    word == "/";  }    /// ---测试代码---  int main() {   assert(reversepolishnotation("a+b*c")() == "a b c * + ");   assert(reversepolishnotation("(a+b)*c-(a+b)/e")() == "a b + c * a b + e / - ");   assert(reversepolishnotation("3*(2-(5+1))")() == "3 2 5 1 + - * ");  // assert(reversepolishnotation("3*((2-(5+1))")() == "3 2 5 1 + - * "); // failed case: redundant '('  // assert(reversepolishnotation("3*(?2-(5+1))")() == "3 2 5 1 + - * "); // failed case: can not recognize '?'   return 0;  }