计算四则运算表达式主要是分两步运算

　　　　　 第一步是把 中缀表达式 转换成 后缀表达式。参考大一下期学的 《》3.3.3 教材，完成下列代码:

 

static String mid_to_suf(String str) {        Stack
       
         s = new Stack
        
         ();        String suf = new String();        HashMap
         
           map = new HashMap
          
           ();        map.put('+', 1); // 设置运算符的优先级        map.put('-', 1);        map.put('*', 2);        map.put('/', 2);        map.put('(', 3);        for (int i = 0; i < str.length(); i++) {            char c = str.charAt(i);            while (c >= '0' && c <= '9') {                suf += c;                if (i + 1 < str.length())                    i++;                c = str.charAt(i);            }            suf += ' ';            if (c == '=')                break;            if (c == ' ')                suf += c;            else {                if (s.isEmpty())                    s.push(c);                else {                    while (!s.isEmpty()) {                        char op = s.peek();                        if (op == '(')                            break;                        if (map.get(op) < map.get(c))                            break;                        else {                            s.pop();                            suf += op;                        }                    }                    s.push(c);                }            }        }        while (!s.isEmpty())            suf += s.pop();        return suf;    }
          
         
        
       

 

　　第二步是计算后缀表达式的方法， 曾在湘大oj上做过这样的题目  

　　

private static Fraction cal(String s) {        Stack
      
        stack = new Stack
       
        ();        for (int i = 0; i < s.length(); i++) {            char c = s.charAt(i);            boolean flag = false;            int tmp = 0;            if (c == ' ')                continue;            while (is_num(c)) {                tmp *= 10;                tmp += (c - '0');                i++;                c = s.charAt(i);                flag = true;            }            if (flag) {                i--;                stack.push(new Fraction(tmp, 1));                tmp = 0;            } else {                Fraction x1 = stack.pop();                Fraction x2 = stack.pop();                Fraction ans = new Fraction(1, 1);                if (c == '*')                    ans = x1.mul(x2);                else if (c == '-')                    ans = x2.sub(x1);                else if (c == '+')                    ans = x1.add(x2);                else if (c == '/')                    ans = x2.div(x1);                stack.push(ans);            }        }        return stack.pop();    }    private static boolean is_num(char c) {        if (c >= '0' && c <= '9')            return true;        return false;    }    static class Fraction {        private int numerator, denominator;        public Fraction(int numerator, int denominator) {            this.numerator = numerator;            this.denominator = denominator;        }        int gcd(int x, int y) {            while (y != 0) {                int z = x % y;                x = y;                y = z;            }            return x;        }        void reduction() {            int tmp = gcd(this.numerator, this.denominator);            this.numerator = this.numerator / tmp;            this.denominator = this.denominator / tmp;        }        Fraction add(Fraction f1) {            int temp = f1.denominator;            f1.denominator = this.denominator * f1.denominator;            f1.numerator = this.numerator * temp + f1.numerator * this.denominator;            f1.reduction();            return f1;        }        Fraction sub(Fraction f1) {            int tmp = f1.denominator;            f1.denominator = this.denominator * f1.denominator;            f1.numerator = this.numerator * tmp - f1.numerator * this.denominator;            f1.reduction();            return f1;        }        Fraction mul(Fraction f1) {            f1.denominator = f1.denominator * this.denominator;            f1.numerator = f1.numerator * this.numerator;            f1.reduction();            return f1;        }        Fraction div(Fraction f1) {            int tmp = f1.denominator;            f1.denominator = this.denominator * f1.numerator;            f1.numerator = this.numerator * tmp;            f1.reduction();            return f1;        }    }
       
      

 

　　因此稍微复杂一点的题目就可以做出来了 不过，中间还有些许 负数什么的，还有文件操作之类的要求，时间紧迫，我会慢慢一步一步完成之后的要求的。

之后是测试样例。。。