Add the solution for building LRU cache from scratch

Author: rayworks

src/main/java/org/sean/array/LRUCacheRaw.java

@@ -0,0 +1,79 @@
+package org.sean.array;
+
+import java.util.HashMap;
+
+public class LRUCacheRaw {
+    static class CacheItem {
+        int key;
+        int value;
+        CacheItem next;
+        CacheItem prev;
+
+        public CacheItem(int key, int value) {
+            this.key = key;
+            this.value = value;
+        }
+    }
+
+    private final int capacity;
+    private final CacheItem head;
+    private final CacheItem tail;
+
+    private final HashMap<Integer, CacheItem> itemHashMap = new HashMap<>();
+
+    public LRUCacheRaw(int capacity) {
+        this.capacity = capacity;
+
+        head = new CacheItem(-1, 0);
+        tail = new CacheItem(10001, 0);
+
+        // doubly linked list + hashMap
+        head.prev = tail;
+        head.next = tail;
+        tail.next = head;
+        tail.prev = head;
+    }
+
+    public int get(int key) {
+        CacheItem item = itemHashMap.get(key);
+        if (item == null) {
+            return -1;
+        }
+        int val = item.value;
+        item.prev.next = item.next;
+        item.next.prev = item.prev;
+
+        insertAfterHead(item);
+
+        return val;
+    }
+
+    private void insertAfterHead(CacheItem item) {
+        item.prev = head;
+        item.next = head.next;
+        head.next.prev = item;
+        head.next = item;
+    }
+
+    public void put(int key, int value) {
+        CacheItem item = itemHashMap.get(key);
+        if (item == null) {
+            if (itemHashMap.size() == capacity) {
+                // remove the last element from the tail
+                CacheItem prevTail = tail.prev;
+                prevTail.prev.next = tail;
+                tail.prev = prevTail.prev;
+
+                itemHashMap.remove(prevTail.key);
+            }
+            item = new CacheItem(key, value);
+            itemHashMap.put(key, item);
+        } else {
+            item.value = value;
+            item.prev.next = item.next;
+            item.next.prev = item.prev;
+        }
+
+        insertAfterHead(item);
+    }
+}