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 这是一个很简单的题目，解答如下 用多个if即可解决
Algorithm:
pop string from the end of the vector
if it's number, just return it
if it's operation, call function recursively for 2nd operand and 1st
int evalRPN(vector<string> &n) { string s = n.back(); n.pop_back(); if ( s== "" || s=="/" || s=="+" || s == "-" ){ int r2 = evalRPN(n); int r1 = evalRPN(n); if ( s=="") return r1*r2; if ( s=="/") return r1/r2; if ( s=="+") return r1+r2; if ( s=="-") return r1-r2; } else return atoi(s.c_str()); }
