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
使用栈(Stack)，如果是操作数则压栈，如果是操作符则弹出栈顶两个元素进行相应运算之后再压栈。
最后栈中剩余元素即计算结果。
注意：操作数的顺序。
class Solution {
public:
int evalRPN(vector<string>& tokens) {
stack<int> stk;
for(int i = ; i < tokens.size(); i ++)
{
if(tokens[i] == "+")
{
int a = stk.top();
stk.pop();
int b = stk.top();
stk.pop();
stk.push(b+a);
}
else if(tokens[i] == "-")
{
int a = stk.top();
stk.pop();
int b = stk.top();
stk.pop();
stk.push(b-a);
}
else if(tokens[i] == "*")
{
int a = stk.top();
stk.pop();
int b = stk.top();
stk.pop();
stk.push(b*a);
}
else if(tokens[i] == "/")
{
int a = stk.top();
stk.pop();
int b = stk.top();
stk.pop();
stk.push(b/a);
}
else
{
int digit = atoi(tokens[i].c_str());
stk.push(digit);
}
}
return stk.top();
}
};