calculator.md

Q

Implement a basic calculator, supporting operators such as +, -, *, / and operands like ( and ).

For example: "(1+2) *10 -25/(1-7)" -> 34

A

I think that maybe there is an error in the example above. The expression should be : (1+2)*10-24/(1-7) -> 34 Here is some implementation, convert infix notation in reverse pollish notation and calculate it

boolean isDigit(char ch) {
      return ch >= '0' && ch <= '9';
  }
  int calc(int op2, int op1, char ch) {
      switch(ch) {
      case '-': return op1 - op2;
      case '+': return op1 + op2;
      case '/': return op1 / op2;
      case '*': return op1 * op2;
      }
      return 0;
  }
  boolean higherPriority(char op1, char op2) {
      if ((op1 =='*') || (op1 =='/')) 
          return true;
      if ((op2 =='+') || (op2 =='-')) 
          return true;
      return false;
  }
  int simpleCalculator(String exp) {
      Stack<Integer> st = new Stack<>();
      Stack<Character> op = new Stack<>();
      int digit = 0;
      boolean hasDigit = false;
      for (int i = 0; i < exp.length(); i++) {
          if (isDigit(exp.charAt(i))) {
              hasDigit = true;
              digit = digit*10 + (exp.charAt(i) - '0');
          } else {
              if(hasDigit) {
                  hasDigit = false;
                  st.push(digit);
                  digit = 0;
              }
              if (exp.charAt(i) == '(') {
                  op.push('(');
              } else if(exp.charAt(i) == ')') {
                  while (op.peek() != '(') {
                      st.push(calc(st.pop(), st.pop(), op.pop()));
                  }
                  op.pop();
              } else {
                   while (!op.isEmpty() && op.peek() != '(' && higherPriority(op.peek(), exp.charAt(i))) {                             
                       st.push(calc(st.pop(), st.pop(), op.pop()));
                   }
                  op.push(exp.charAt(i));
              }
          }
      }
      if(hasDigit)
          st.push(digit);
      while(!op.isEmpty()) {
          st.push(calc(st.pop(), st.pop(), op.pop()));
      }
      return st.peek();
  }
}