POJ-1539（表达式求值）

#include <cctype>
#include <string>
#include <iostream>
using namespace std;

int  value[26];
char used[26], post[26];

void preEval(string& exp)
{
int i = 0, len = exp.size();
for(int k = 0; k < len; ++k){
if(!isspace(exp[k])) exp[i++] = exp[k];
}
exp.resize(len = i);
for(i = 0; i < 26; ++i){
used[i] = 0;
value[i] = i + 1;
post[i] = 0;
}
for(i = 0; i < len; ++i){
if(isalpha(exp[i])) used[exp[i] - 'a'] = 1;
}
for(i = 0; i < len && (i = exp.find("++", i)) != string::npos; ){
if(i + 2 < len && isalpha(exp[i+2]))
++value[exp[i+2] - 'a'];
else
post[exp[i-1] - 'a'] = 1;
exp = exp.substr(0, i) + exp.substr(i + 2);
}
for(i = 0; i < len && (i = exp.find("--", i)) != string::npos; ){
if(i + 2 < len && isalpha(exp[i+2]))
--value[exp[i+2] - 'a'];
else
post[exp[i-1] - 'a'] = -1;
exp = exp.substr(0, i) + exp.substr(i + 2);
}
//    cout << "exp = " << exp << "\n";
}
int eval(const string& exp, int res = 0)
{
if(exp.empty()) return res;

int i = 0;
bool add = true;
if(!isalpha(exp[i])){
add = exp[i] == '+';
++i;
}
if(add) res += value[exp[i] - 'a'];
else res -= value[exp[i] - 'a'];
return eval(exp.substr(i+1), res);
}
void postEval()
{
for(int i = 0; i < 26; ++i){
if(used[i]) cout << "    "
<< char('a' + i) << " = "
<< value[i] + post[i] << "\n";
}
}

int main()
{
ios::sync_with_stdio(false);
string exp;
while(getline(cin, exp)){
cout << "Expression: " << exp << "\n";
preEval(exp);
cout << "    " << "value = " << eval(exp) << "\n";
postEval();
}
return 0;
}