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
思考:注意负数。
class Solution { public: int stoi(string str) { bool flag=true; if(str[0]=='-') flag=false; int ret=0,i; i=(flag)?0:1; for(;i<str.size();i++) { ret=ret*10+(str[i]-'0'); } return (flag)?ret:(-ret); } int evalRPN(vector<string> &tokens) { int ans=0; stack<int> s; int a,b,c; for(int i=0;i<tokens.size();i++) { if(tokens[i]!="+"&&tokens[i]!="-"&&tokens[i]!="*"&&tokens[i]!="/") { string str=tokens[i]; int temp=stoi(str); s.push(temp); } else { a=s.top(); s.pop(); b=s.top(); s.pop(); if(tokens[i]=="+") c=b+a; if(tokens[i]=="-") c=b-a; if(tokens[i]=="*") c=b*a; if(tokens[i]=="/") c=b/a; s.push(c); } } ans=s.top(); return ans; } };