LRU Cache added.

Design/LRUCache.swift

@@ -0,0 +1,75 @@
+/**
+ * Question Link: https://leetcode.com/problems/lru-cache/
+ * Primary idea: Use Doubly linked list and hash table to build the LRU cache.
+ * Time Complexity: O(1), Space Complexity: O(n)
+ *
+ */
+
+class DoublyLinkedList{
+    var key: Int
+    var value: Int
+    var previous: DoublyLinkedList?
+    var next: DoublyLinkedList?
+    var hashValue: Int
+
+    init(_ key: Int, _ value: Int) {
+        self.key = key
+        self.value = value
+        self.hashValue = key
+    }
+}
+
+class LRUCache{
+    var maxCapacity: Int
+    var head: DoublyLinkedList
+    var tail: DoublyLinkedList
+    var cache: [Int: DoublyLinkedList]
+
+    init(_ maxCapacity: Int) {
+        self.maxCapacity = maxCapacity
+        self.cache = [Int: DoublyLinkedList]()
+        self.head = DoublyLinkedList(Int.min, Int.min)
+        self.tail = DoublyLinkedList(Int.max, Int.max)
+        self.head.next = self.tail
+        self.tail.previous = self.head
+    }
+
+    func add(_ node: DoublyLinkedList){
+        let next = head.next
+        head.next = node
+        node.previous = head
+        node.next = next
+        next?.previous = node
+    }
+
+    func remove(_ node: DoublyLinkedList){
+        let previous = node.previous
+        let next = node.next
+        previous?.next = next
+        next?.previous = previous
+    }
+
+    func get(_ key: Int) -> Int{
+        if let node = cache[key]{
+            remove(node)
+            add(node)
+            return node.value
+        }
+        return -1
+    }
+
+    func put(_ key: Int, _ value: Int){
+        if let node = cache[key]{
+            remove(node)
+            cache.removeValue(forKey: key)
+        }else if cache.keys.count >= maxCapacity{
+            if let leastNode = tail.previous{
+                remove(leastNode)
+                cache.removeValue(forKey: leastNode.key)
+            }
+        }
+        let newNode = DoublyLinkedList(key, value)
+        cache[key] = newNode
+        add(newNode)
+    }
+}

README.md

@@ -348,6 +348,11 @@
 [Graph Valid Tree](https://leetcode.com/problems/graph-valid-tree/)| [Swift](./UnionFind/GraphValidTree.swift)| Medium| O(nlogn)| O(n)|
 [Number of Islands II](https://leetcode.com/problems/number-of-islands-ii/)| [Swift](./UnionFind/NumberIslandsII.swift)| Hard| O(klogmn)| O(mn)|
 
+## Design
+| Title | Solution | Difficulty | Time | Space |
+| ----- | -------- | ---------- | ---- | ----- |
+[LRU Cache](https://leetcode.com/problems/lru-cache/)| [Swift](./Design/LRUCache.swift)| Hard| O(1)| O(n)|
+
 ## Google
 | Title | Solution | Difficulty | Frequency |
 | ----- | -------- | ---------- | --------- |