basic-calculator.cc

// Basic Calculator
// operator-precedence parser
class Solution
{
public:
  int calculate(string s) {
    stack<int> xs;
    stack<char> ops;
    map<char, int> isp, icp;
    isp['\0'] = 0; isp['+'] = 3; isp['-'] = 3; isp['('] = 1;
    icp['\0'] = 0; icp['+'] = 2; icp['-'] = 2; icp['('] = 4; icp[')'] = 1;
    ops.push('\0');
    for (size_t i = 0; i <= s.size(); ) {
      char op = i == s.size() ? '\0' : s[i];
      if (op == ' ')
        i++;
      else if (isdigit(op)) {
        int x = 0;
        do x = x*10+s[i]-'0';
        while (isdigit(s[++i]));
        xs.push(x); // shift
      } else {
        i++;
        int x, y;
        for (; isp[ops.top()] > icp[op]; ops.pop()) {
          y = xs.top(); xs.pop();
          x = xs.top(); xs.pop();
          xs.push(ops.top() == '+' ? x+y : x-y); // reduce, X '+' Y => X+Y  or  X '-' Y => X-Y
        }
        if (isp[ops.top()] == icp[op]) // reduce, '(' X ')' => X  or  '\0' X '\0' => X
          ops.pop();
        else
          ops.push(op); // shift
      }
    }
    return xs.top();
  }
};