150. Evaluate Reverse Polish Notation 逆波兰式

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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

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分析：

实现逆波兰式。

用栈就可以，注意数字可能是负数。

代码：

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> num;
        stack<char> t;
        for(int i=0;i<tokens.size();++i)
        {
            if(("+"==tokens[i])||("-"==tokens[i])||("*"==tokens[i])||("/"==tokens[i]))
            {
                int t2=num.top();num.pop();
                int t1=num.top();num.pop();
                if("+"==tokens[i]) num.push(t1+t2);
                else if("-"==tokens[i]) num.push(t1-t2);
                else if("*"==tokens[i]) num.push(t1*t2);
                else if("/"==tokens[i]) num.push(t1/t2);
            }
            else
            {
                int temp=0;
                string s=tokens[i];
                for(int j=0;j<s.size();++j)
                {
                    if(('0'<=s[j] && s[j]<='9'))
                    temp=temp*10+(s[j]-'0');
                }
                if(s[0]=='-') temp=temp*(-1);
                // temp=(s[0]=='-')?(-1*temp)::temp;
                num.push(temp);
            }
        }
        return num.top();
    }
};

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