Merge pull request neetcode-gh#615 from diss1993/ticket/150

Swift | 150. Evaluate Reverse Polish Notation

swift/150-Evaluate-Reverse-Polish-Notation.swift

@@ -0,0 +1,58 @@
+class Solution {
+ func evalRPN(_ tokens: [String]) -> Int {
+ guard tokens.count > 0 else { return 0 }
+ var stack: Stack<Int> = Stack()
+ for c in tokens {
+ if c == "+" {
+ stack.push((stack.pop() ?? 0) + (stack.pop() ?? 0) )
+ } else if c == "-" {
+ var a = stack.pop() ?? 0
+ var b = stack.pop() ?? 0
+ stack.push(b - a)
+ } else if c == "*" {
+ stack.push((stack.pop() ?? 0) * (stack.pop() ?? 0) )
+ } else if c == "/" {
+ var a = stack.pop() ?? 0
+ var b = stack.pop() ?? 0
+ stack.push(b / a)
+ } else {
+ stack.push(Int(c) ?? 0)
+ }
+ }
+ return stack.pop() ?? 0
+ }
+}
+
+class Node<T> {
+ var data: T
+ var next: Node<T>?
+ init(_ value: T) {
+ self.data = value
+ }
+}
+
+class Stack<T> {
+
+ var head: Node<T>?
+
+ var isEmpty: Bool {
+ head == nil
+ }
+
+ func peak() -> Node<T>? {
+ head
+ }
+
+ func push(_ data: T) {
+ var node = Node<T>(data)
+ node.next = head
+ head = node
+ }
+
+ func pop() -> T? {
+ var data = head?.data
+ head = head?.next
+ return data
+ }
+
+}