JZTXT

字符串中四则运算表达式求值

发布时间 2023-04-12 21:12:33作者: MarkLeeBYR

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<>(); //存储计算结果
        Stack<String> opStack = new Stack<>(); // 存储运算符符
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (ch == ' ') { // 空字符，跳过
                continue;
            }
            if (ch >= '0' && ch <= '9') { //是数字，需要加到sb的末尾
                sb.append(ch);
            } else {
                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);
                } else {
                    while (!opStack.isEmpty()) {
                        if (helper(op, opStack.peek())){ // 比较计算运算符的优先级
                            calFun(numStack, opStack); // 计算结果
                            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();
    }