Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

1 int evalRPN(vector
     
      & tokens) 2     {
   //字符串可以直接比较。。不用strcmp 直接tokens[i]=="*" 3         stack
      
        st; 4         for (int i = 0; i < tokens.size();i++) 5         { 6             if (atoi(tokens[i].c_str())!=0)st.push(atoi(tokens[i].c_str()));//返回非0代表为数字  7             else if (strcmp(tokens[i].c_str(), "0") == 0) 8             { 9                  st.push(atoi(tokens[i].c_str()));                10             }11             else12             {                13                 int b = st.top();14                 st.pop();15                 int a = st.top();16                 st.pop();17                 if (strcmp(tokens[i].c_str(), "+")==0)    st.push(a + b);                18                 else if (strcmp(tokens[i].c_str(), "-") == 0)st.push(a - b);19                 else if (strcmp(tokens[i].c_str(), "*") == 0)st.push(a * b);20                 else if (tokens[i] == "/")st.push(a / b);21                 else22                 {23                     //说明有别的字符 错误                    24                 }25             }26         }27         return st.top();28     }
      
     

 

1  int evalRPN(vector
     
       &tokens) { 2         stack
      
        st; 3         int s1,s2; 4         s1=s2=0; 5         int res=0; 6         for(vector
       
        ::iterator iter=tokens.begin();iter!=tokens.end();iter++) 7         { 8                 if (*iter == "+") 9                 {10                     s1=st.top();11                     st.pop();12                     s2=st.top();13                     st.pop();14                    res=s1+s2;15                    st.push(res);16                 }17                    18                 else if (*iter == "-")19                 {20                     s1=st.top();21                     st.pop();22                     s2=st.top();23                     st.pop();24                    res=s2-s1;25                    st.push(res);26                 }27                 else if (*iter == "*")28                 {29                     s1=st.top();30                     st.pop();31                     s2=st.top();32                     st.pop();33                    res=s1*s2;34                    st.push(res);35                 }36                 else if (*iter== "/")37                 {38                     s1=st.top();39                     st.pop();40                     s2=st.top();41                     st.pop();42                     res=s2/s1;43                     st.push(res);44                 }45                 else 46                 {47                     st.push(atoi((*iter).c_str()));48                 }49             }50             return st.top(); 51             53         }