Leetcode--Evaluate Reverse Polish Notation

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

<span style="color:#333333;">class Solution {
public:
int evalRPN(vector<string> &tokens) {
stack<int> stk;
int len=tokens.size();
if(len<=0)
return 0;
else if(len==1)
return atoi(tokens[0].c_str());
for(int i=0;i<len;i++)
{
if(tokens[i][0]>='0'&&tokens[i][0]<='9')
stk.push(atoi(tokens[i].c_str()));
else if(tokens[i][0]=='-'&&tokens[i].size()>1)//</span><span style="color:#ff0000;">特别注意区分负数和负号</span><span style="color:#333333;">
stk.push(atoi(tokens[i].c_str()));
else if(tokens[i][0]=='+'||tokens[i][0]=='-'||tokens[i][0]=='*'||tokens[i][0]=='/'){
int op1=stk.top();
stk.pop();
int op2=stk.top();
stk.pop();
switch(tokens[i][0]){
case '+': stk.push(op2+op1);break;
case '-': stk.push(op2-op1);break;
case '*': stk.push(op2*op1);break;
case '/': stk.push(op2/op1);break;
}
}
}
return stk.top();
}
};</span>