Evaluate Reverse Polish Notation LeetCode Solution

Problem – Evaluate Reverse Polish Notation

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

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

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

Example 1:

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

Example 2:

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

Example 3:

Input: tokens = ["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

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Evaluate Reverse Polish Notation LeetCode Solution in C++

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        unordered_map<string, function<int (int, int) > > map = {
            { "+" , [] (int a, int b) { return a + b; } },
            { "-" , [] (int a, int b) { return a - b; } },
            { "*" , [] (int a, int b) { return a * b; } },
            { "/" , [] (int a, int b) { return a / b; } }
        };
        std::stack<int> stack;
        for (string& s : tokens) {
            if (!map.count(s)) {
                stack.push(stoi(s));
            } else {
                int op1 = stack.top();
                stack.pop();
                int op2 = stack.top();
                stack.pop();
                stack.push(map[s](op2, op1));
            }
        }
        return stack.top();
    }
};

Evaluate Reverse Polish Notation LeetCode Solution in Java

import java.util.Stack;

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

Evaluate Reverse Polish Notation LeetCode Solution in Python

class Solution(object):
    def evalRPN(self, tokens):
        stack = []
        for t in tokens:
            if t not in "+-*/":
                stack.append(int(t))
            else:
                r, l = stack.pop(), stack.pop()
                if t == "+":
                    stack.append(l+r)
                elif t == "-":
                    stack.append(l-r)
                elif t == "*":
                    stack.append(l*r)
                else:
                    stack.append(int(float(l)/r))
        return stack.pop()

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