Evaluate Reverse Polish Notation(逆波兰式)

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

逆波兰式，不用说，肯定考虑栈。

主要是题目所给的是字符串的数组，需要多次进行数字到字符串或者字符串到数字的转换，具体实现参考我的blog，整数转字符串，字符串转整数。

这里我采用的是c++标准库sstream来实现转换。

代码：

class Solution {
private:
bool isSymbol(string a){
return a=="+"||a=="-"||a=="*"||a=="/";
}
int Evaluate(int a,int b,char c){
switch (c)
{
case '+':
return a+b;
break;
case '-':
return a-b;
break;
case '*':
return a*b;
break;
case '/':
return a/b;
break;
default:
break;
}
}
public:
int evalRPN(vector<string> &tokens) {
stack<string> container;
for(int i=0;i<tokens.size();++i){
if(isSymbol(tokens[i])&&!container.empty()){
string temp2Str=container.top();container.pop();
string temp1Str=container.top();container.pop();
int temp2;
int temp1;
stringstream s;
s<<temp2Str;s>>temp2;
s.clear();
s<<temp1Str;s>>temp1;
s.clear();

stringstream s2;
int res=Evaluate(temp1,temp2,tokens[i][0]);
s2<<res;
string resStr=s2.str();
container.push(resStr);
}else{
container.push(tokens[i]);
}
}
stringstream s;
int result=0;
string reultStr=container.top();
s<<reultStr;
s>>result;
return result;
}
};