LeetCode [150] Evaluate Reverse Polish Notation

 150. Evaluate Reverse Polish Notation

Medium

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:

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

Example 2:

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

Example 3:

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

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class Solution { public: int evalRPN(vector<string>& tokens) { stack<int> stk; int n = tokens.size(); for(int i=0; i<n; ++i){ string s = tokens[i]; if(s=="+"||s=="-"||s=="*"||s=="/"){ int rhs = stk.top(); stk.pop(); int lhs = stk.top(); stk.pop(); int res; if(s=="+") res = lhs+rhs; else if(s=="-") res = lhs-rhs; else if(s=="*") res = lhs*rhs; else if(s=="/") res = lhs/rhs; stk.push(res); }else{ stk.push(stoi(s)); } } return stk.top(); } };

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class Solution { public int evalRPN(String[] tokens) { Stack<Integer> stk = new Stack<>(); for(String s : tokens){ if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")){ int b = stk.pop(); int a = stk.pop(); int r = 0; if(s.equals("+")) r = a+b; else if(s.equals("-")) r = a-b; else if(s.equals("*")) r = a*b; else r=a/b; stk.add(r); }else{ stk.push(Integer.parseInt(s)); } } return stk.pop(); } }