字符串中四则运算表达式求值

字符串内容：1 + 2 * 5 - 6 / 2

写代码求出该字符串的值

 

class Main {
  private static final Map<String, Integer> map = new HashMap<>();

  private void initMap() {
    map.put("+", 1);
    map.put("-", 1);
    map.put("*", 2);
    map.put("/", 2);
  }

  public int cal(String str) {
    if (str == null || str.length() == 0) {
      return 0;
    }
    initMap();
    Stack<Integer> numStack = new Stack<>();
    Stack<String> opStack = new Stack<>();
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < str.length(); i++) {
      char ch = str.charAt(i);
      if (ch == ' ') {
        continue;
      }
      if (ch >= '0' && ch <= '9') {
        sb.append(ch);
        continue;
      }
      if (sb.length() > 0) {
        numStack.push(Integer.valueOf(sb.toString()));
        sb.delete(0, sb.length());
      }
      String op = String.valueOf(ch);
      if (opStack.isEmpty()) {
        opStack.push(op);
        continue;
      }
      while (true) {
        // 如果当前操作符是低级操作符，则把stack存的前面元素取出来计算。如果当前操作符的优先级比stack顶端操作符的更高，直接压入栈
        if (helper(op, opStack.peek())) {
          calFun(numStack, opStack);
          if (opStack.isEmpty()) {
            opStack.push(op);
            break;
          }
        } else {
          opStack.push(op);
          break;
        }
      }
    }

    if (sb.length() > 0) {
      numStack.push(Integer.valueOf(sb.toString()));
    }
    while (!opStack.empty()) {
      calFun(numStack, opStack);
    }
    return numStack.pop();
  }

  private void calFun(Stack<Integer> numStack, Stack<String> opStack) {
    String op = opStack.pop();
    int var1 = numStack.pop();
    int var2 = numStack.pop();
    int result = 0;
    switch (op) {
      case "+":
        result = var1 + var2;
        break;
      case "-":
        result = var2 - var1;
        break;
      case "*":
        result = var1 * var2;
        break;
      case "/":
        result = var2 / var1;
        break;
    }
    numStack.push(result);
  }

  private boolean helper(String op1, String op2) {
    return map.get(op1) <= map.get(op2);
  }

  public static void main(String[] args) {
    String s1 = "1 + 2 * 5 - 6 / 2";
    Main main = new Main();
    int res = main.cal(s1);
    System.out.println(res);
  }
}