[Stack] add solution to Evaluate Reverse Polish Notation and Valid Pa…

…rentheses

Stack/EvaluateReversePolishNotation.swift

@@ -0,0 +1,50 @@
+/**
+ * Question Link: https://leetcode.com/problems/evaluate-reverse-polish-notation/
+ * Primary idea: Push a number to a stack and pop two for operation when encounters a operator
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class EvaluateReversePolishNotation {
+    func evalRPN(tokens: [String]) -> Int {
+        var stack = [Int]()
+
+        for str in tokens {
+            if _isNumber(str) {
+                stack.append(Int(str)!)
+            } else {
+                guard stack.count > 1 else {
+                    return 0
+                }
+
+                let post = stack.removeLast()
+                let prev = stack.removeLast()
+
+                let res = _operate(prev, post, str)
+                stack.append(res)
+            }
+        }
+
+        if stack.count == 0 {
+            return 0
+        } else {
+            return stack.first!
+        }
+    }
+
+    private func _isNumber(str: String) -> Bool {
+        return Int(str) != nil
+    }
+
+    private func _operate(prev: Int, _ post: Int, _ token: String) -> Int{
+        switch token {
+            case "+":
+                return prev + post
+            case "-":
+                return prev - post
+            case "*":
+                return prev * post
+            default:
+                return prev / post
+       }
+    }
+}

Stack/ValidParentheses.swift

@@ -0,0 +1,31 @@
+/**
+ * Question Link: https://leetcode.com/problems/valid-parentheses/
+ * Primary idea: Use a stack to see whether the peek left brace is correspond to the current right one
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class ValidParentheses {
+    func isValid(s: String) -> Bool {
+        var stack = [Character]()
+
+        for char in s.characters {
+            if char == "(" || char == "[" || char == "{" {
+                stack.append(char)
+            } else if char == ")" {
+                guard stack.count != 0 && stack.removeLast() == "(" else {
+                    return false
+                }
+            } else if char == "]" {
+                guard stack.count != 0 && stack.removeLast() == "[" else {
+                    return false
+                }
+            } else if char == "}" {
+                guard stack.count != 0 && stack.removeLast() == "{" else {
+                    return false
+                }
+            }
+        }
+
+        return stack.count == 0
+    }
+}