Evaluate Reverse Polish Notation

Desicription

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

  • Division between two integers should truncate toward zero.
  • The given RPN expression is always valid. That means the expression would always evaluate to a result and there won’t be any divide by zero operation.

Example 1:

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Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

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Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

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Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

Solution

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class Solution {
public:
int evalRPN(vector<string>& tokens) {
string s = tokens.back();
tokens.pop_back();
if(s == "*" || s == "/" || s == "+" || s == "-") {
int r2 = evalRPN(tokens);
int r1 = evalRPN(tokens);
if(s == "*")
return r1 * r2;
else if(s == "/")
return r1 / r2;
else if(s == "+")
return r1 + r2;
else if(s == "-")
return r1 - r2;
}
else
return atoi(s.c_str());
}
};