算法设计与应用基础系列12



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:

1. pop string from the end of the vector

2. if it's number, just return it

3. 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());
   }