1 #include<stack>
 2 #include<iostream>
 3 #include<string>
 4 using namespace std;
 5 int mat[5][6] = {//+,-,*,/,(
 6     1, 1, 0, 0, 0, 1,
 7     1, 1, 0, 0, 0, 1,
 8     1, 1, 1, 1, 0, 1,
 9     1, 1, 1, 1, 0, 1,
10     0, 0, 0, 0, 0, 1
11 };
12 string str;
13 stack<int> nu;
14 stack<int> op;
15 int len;
16 void get(bool &flag, int &retn, int &i){//返回字符串的第i个数据 
17     char tmp;
18     tmp = str.at(i);
19     if (tmp <= ‘9‘&&tmp >= ‘0‘){//是数字 
20         int x = 0;
21         while (i<len&&str.at(i) <= ‘9‘&&str.at(i) >= ‘0‘){
22             x = x * 10 + str.at(i) - 48;
23             i++;
24         }
25         i--;
26         flag = false;
27         retn = x;
28     }
29     else{//是运算符 
30         flag = true;
31         if (tmp == ‘+‘) retn = 0;
32         else if (tmp == ‘-‘) retn = 1;
33         else if (tmp == ‘*‘) retn = 2;
34         else if (tmp == ‘/‘) retn = 3;
35         else if (tmp == ‘(‘) retn = 4;
36         else retn = 5;
37     }
38 }
39 int main(){
40     cin >> str;
41     len = str.size();
42     int idx = 0;
43     bool isstring;
44     int retnum;
45     while (idx<len){
46         get(isstring, retnum, idx);
47         if (isstring == false){//是数字 
48             nu.push(retnum);
49         }
50         else{//是操作符 
51             if (op.empty() || mat[op.top()][retnum] == 0){//符号入栈 
52                 op.push(retnum);
53             }
54             else{//需要进行出栈 
55                 while (!op.empty() && mat[op.top()][retnum] == 1){//如果符号栈非空并且 需要进行出栈操作 
56                     int tmp = op.top();//栈顶符号 
57                     op.pop();
58                     if (tmp == 4) break;//如果栈顶是左括号，直接左括号出栈。
59                     //如果栈顶不是左括号，取出数字栈的最上面两个 和运算符运算。 
60                     int a = nu.top();
61                     nu.pop();
62                     int b = nu.top();
63                     nu.pop();
64                     if (tmp == 0) nu.push(a + b);
65                     else if (tmp == 1) nu.push(b-a);
66                     else if (tmp == 2) nu.push(a*b);
67                     else if (tmp == 3) nu.push(b / a);
68                 }
69                 if (retnum != 5) op.push(retnum);
70             }
71         }
72         idx++;
73     }
74     while (!op.empty()){//最后依次取出符号栈来进行运算
75         int tmp = op.top();//栈顶符号 
76         op.pop();
77         int a = nu.top();
78         nu.pop();
79         int b = nu.top();
80         nu.pop();
81         if (tmp == 0) nu.push(a + b);
82         else if (tmp == 1) nu.push(b-a);
83         else if (tmp == 2) nu.push(a*b);
84         else if (tmp == 3) nu.push(b/a);
85     }
86     cout << nu.top() << endl;
87 }