150. 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
【思路】建立一个栈，非符号压入栈，遇到符号，将栈中最顶的两个数操作后值再压入栈，最后返回栈顶元素。
class Solution {
public:
int evalRPN(vector<string>& tokens) {
stack<int> stk;
if(tokens.size()==0) return 0;
if(tokens.size()==1) return atoi(tokens[0].c_str());
for(int i =0; i < tokens.size(); i++)
{
int tmp1,tmp2;
if(tokens[i]!="+"&& tokens[i]!="-" && tokens[i]!="*" && tokens[i]!="/")
{
stk.push(atoi(tokens[i].c_str()));
}
else if(tokens[i]=="+")
{
tmp1 = stk.top();
stk.pop();
tmp2 = stk.top();
stk.pop();
stk.push(tmp1 + tmp2);
}
else if(tokens[i]=="-"){
tmp1 = stk.top();
stk.pop();
tmp2 = stk.top();
stk.pop();
stk.push(tmp2 - tmp1);
}
else if(tokens[i]=="*")
{
tmp1 = stk.top();
stk.pop();
tmp2 = stk.top();
stk.pop();
stk.push(tmp2 * tmp1);
}
else if(tokens[i]=="/")
{
tmp1 = stk.top();
stk.pop();
tmp2 = stk.top();
stk.pop();
stk.push(tmp2 / tmp1);
}
}
return stk.top();
}
};