150. Evaluate Reverse Polish Notation

Author: Partho Biswas

README.md

@@ -249,6 +249,7 @@ I have solved quite a number of problems from several topics. See the below tabl
 |04| [394. Decode String](https://leetcode.com/problems/decode-string/)| [Python](https://tinyurl.com/wu6rdaw/394_Decode_String.py)| --- | Medium | 📌 Classic stack problem |
 |05| [239. Sliding Window Maximum](https://tinyurl.com/rvhujl5)| [Python](https://tinyurl.com/wu6rdaw/239_Sliding_Window_Maximum.py)| **[Video 1](https://tinyurl.com/v767bl3)**, [Official](https://tinyurl.com/www4np2), [Art 1](https://tinyurl.com/y8nbroux), [Art 2](https://tinyurl.com/s62fzls), **[Art 3](https://tinyurl.com/vd3dtch)**, **[Art 4](https://tinyurl.com/yx3sjp46)** | Hard | 📌 Can be solved using Heap, Deque and DP |
 |06| [739. Daily Temperatures](https://tinyurl.com/ybbezzmt)| [Python](https://tinyurl.com/wu6rdaw/739_Daily_Temperatures.py)| **[Video 1](https://tinyurl.com/sdbkh3z)**, **[Video 2](https://tinyurl.com/rbvaozl)**, [Art 1](https://tinyurl.com/ss3d9tx) | Medium | 📌 TODO: Check again. A tricky one |
+|07| **[150. Evaluate Reverse Polish Notation](https://tinyurl.com/y2xrje8v)** | [Python](https://tinyurl.com/wu6rdaw/150_Evaluate_Reverse_Polish_Notation.py), [Swift](https://tinyurl.com/wuja3c4/150_Evaluate_Reverse_Polish_Notation.swift)| [Vid 1](https://tinyurl.com/s92fp4r) | Medium | --- |
 
 </p>
 </details>

leetcode.com/python/150_Evaluate_Reverse_Polish_Notation.py

@@ -0,0 +1,32 @@
+# https://tinyurl.com/r35ffgt
+from collections import deque
+class Solution(object):
+    def evalRPN(self, tokens):
+        """
+        :type tokens: List[str]
+        :rtype: int
+        """
+        tokensDeque = deque(tokens)
+        stack = []
+        opes = ["+", "-", "*", "/"]
+        while tokensDeque:
+            currentToken = tokensDeque.popleft()
+            if currentToken in opes:
+                firstNum = int(stack.pop())
+                secondNum = int(stack.pop())
+                result = 0
+                if currentToken == "+":
+                    result = secondNum + firstNum
+                elif currentToken == "-":
+                    result = secondNum - firstNum
+                elif currentToken == "*":
+                    result = secondNum * firstNum
+                else:
+                    if firstNum*secondNum < 0 and secondNum%firstNum != 0:
+                        result = secondNum // firstNum + 1
+                    else:
+                        result = secondNum // firstNum
+                stack.append(str(result))
+            else:
+                stack.append(currentToken)
+        return stack[-1]

leetcode.com/swift/DS_ALGO_PRAC.playground/Sources/swift/150_Evaluate_Reverse_Polish_Notation.swift

@@ -0,0 +1,34 @@
+import Foundation
+
+class Solution {
+    func evalRPN(_ tokens: [String]) -> Int {
+        var opes = ["+", "-", "*", "/"]
+        var stack = [String]()
+        for token in tokens {
+            if opes.contains(token)  {
+                var rightOperand: Double! = Double(stack.popLast()!)
+                var leftOperand: Double! = Double(stack.popLast()!)
+                var result = 0.0
+                if token == "+" {
+                    result = leftOperand + rightOperand
+                } else if token == "-" {
+                    result = leftOperand - rightOperand
+                } else if token == "*" {
+                    result = leftOperand * rightOperand
+                } else {
+                    if (rightOperand*leftOperand < 0) && (Int(leftOperand)%Int(rightOperand) != 0) {
+                        result = leftOperand / rightOperand + 1
+                    } else {
+                        result = leftOperand / rightOperand
+                    }
+                }
+                var res = Int(result.round(.towardZero)
+                stack.append(String(res))
+            } else {
+                stack.append(token)
+            }
+        }
+        print("stack: \(stack)")
+        return Int(stack.popLast()!)!
+    }
+}