Merge pull request #29 from f88083/150-Evaluate-Reverse-Polish-Notation

[Solution ]150. Evaluate Reverse Polish Notation in Java from Leetcode

LeetCode/150-Evaluate-Reverse-Polish-Notation/Java/README.md

@@ -0,0 +1,5 @@
+# How to Run?
+
+1. Go to the leetcode question site. [Link](https://leetcode.com/problems/evaluate-reverse-polish-notation/)
+2. Copy and paste the code.
+3. Run or Submit to see the result.

LeetCode/150-Evaluate-Reverse-Polish-Notation/Java/Solution.java

@@ -0,0 +1,29 @@
+class Solution {
+    public int evalRPN(String[] tokens) {
+        // Init. stack for the integers
+        Stack<Integer> stack = new Stack<>();
+
+        // Iterate through all the tokens
+        for (String token : tokens) {
+            // Check symbol
+            if (token.equals("+")) {
+                // Push the result to the stack
+                stack.push(stack.pop() + stack.pop());
+            } else if (token.equals("-")) {
+                int b = stack.pop();
+                int a = stack.pop();
+                stack.push(a - b);
+            } else if (token.equals("*")) {
+                stack.push(stack.pop() * stack.pop());
+            } else if (token.equals("/")) {
+                int b = stack.pop();
+                int a = stack.pop();
+                stack.push(a / b);
+            } else { // If it is a number, parse it and push to the stack
+                stack.push(Integer.parseInt(token));
+            }
+        }
+        // Pop the result
+        return stack.pop();
+    }
+}

LeetCode/150-Evaluate-Reverse-Polish-Notation/PROBLEM.md

@@ -0,0 +1,45 @@
+# [150. Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation/description/)
+
+You are given an array of strings tokens that represents an arithmetic expression in a [Reverse Polish Notation](http://en.wikipedia.org/wiki/Reverse_Polish_notation).
+
+Evaluate the expression. Return an integer that represents the value of the expression.
+
+**Note** that:
+
+* The valid operators are `'+'`, `'-'`, `'*'`, and `'/'`.
+* Each operand may be an integer or another expression.
+* The division between two integers always **truncates toward zero**.
+* There will not be any division by zero.
+* The input represents a valid arithmetic expression in a reverse polish notation.
+* The answer and all the intermediate calculations can be represented in a **32-bit** integer.
+
+
+## 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].