Evaluate Reverse Polish Notation (leetcode)

题目：

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

题目来源：https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/

解题思路：用栈来保存输入的数据，每遇到一个符号，则弹出两个数据，然后把计算结果入栈，知道vector遍历完。

#include<iostream>
#include<vector>
#include<string>
#include<stack>
using namespace std;

void calculate(char ch,stack<int> &s)
{
int first=s.top();
s.pop();
int second=s.top();
s.pop();
switch(ch)
{
case '+':
s.push(first+second);
break;
case '-':
s.push(second-first);
break;
case '*':
s.push(first*second);
break;
case '/':
s.push(second/first);
break;
}
}

int evalRPN(vector<string> &tokens)
{
if(tokens.empty())
return 0;
stack<int> s;
s.push(atoi(tokens[0].c_str()));
for(int i=1;i<tokens.size();i++)
{
bool flag=0;
if(tokens[i].size()==1)
switch(tokens[i][0])
{
case '+':case'-':case '*':case '/':
flag=1;
calculate(tokens[i][0],s);
break;
default:
break;
}
if(flag==0)
s.push(atoi(tokens[i].c_str()));
}
return s.top();
}

int main()
{
string p[5]={"-1","1","*","-1","+"};
vector<string> tokens(p,p+5);
int result=evalRPN(tokens);

system("pause");
return 0;
}