https://leetcode.com/problems/lru-cache

interview_prep/algorithm/java/ide_handicapped/lru_cache/LRUCache.java

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+class LRUCache {
+
+    //lookup in constant time
+    private java.util.Map<Integer, CacheEntry> cache = new java.util.HashMap<>();
+    private int capacity;
+    // maintain an ordered list that quickly shuffle on access
+    private CacheEntry mostRecently;
+    private CacheEntry leastRecently;
+
+    public LRUCache(int capacity) {
+        this.capacity = capacity;
+        this.cache = new java.util.HashMap<>();
+        this.mostRecently = new CacheEntry(0, 0);
+        this.leastRecently = new CacheEntry(0, 0);
+
+        this.mostRecently.prev = this.leastRecently;
+        this.leastRecently.next = this.mostRecently;
+    }
+
+    public int get(int key) {
+        if(cache.containsKey(key)){
+            CacheEntry entry = cache.get(key);
+            remove(entry);
+            insert(entry);
+            return entry.val;
+        }
+        return -1;
+    }
+
+    public void put(int key, int value) {
+         if(cache.containsKey(key)){
+            CacheEntry entry = cache.get(key);
+            remove(entry);
+        }
+        CacheEntry refresh = new CacheEntry(key, value);
+        cache.put(key, refresh);
+        insert(refresh);
+
+        if(cache.size()> capacity){
+            CacheEntry oldest = this.leastRecently.next;
+            remove(oldest);
+            cache.remove(oldest.key);
+        }
+
+    }
+
+    private void remove(CacheEntry entry){
+        CacheEntry myPre = entry.prev;
+        CacheEntry myNexEntry = entry.next;
+
+        myPre.next = myNexEntry;
+        myNexEntry.prev = myPre;
+    }
+
+    private void insert(CacheEntry entry){
+        CacheEntry prev = this.mostRecently.prev;
+        prev.next = entry;
+        entry.prev = prev;
+        this.mostRecently.prev = entry;
+        entry.next = this.mostRecently;
+    }
+
+    public static void main(String[] args) {
+        LRUCache cache = new LRUCache(5);
+        cache.put(1, 1);
+        cache.put(2, 2);
+        cache.put(3, 3);
+        cache.put(4, 4);
+        cache.put(5, 5);
+        cache.put(6, 6);
+        cache.put(7, 7);
+
+        assert cache.cache.size() == 5;
+        assert cache.get(1) == -1;
+        assert cache.get(2) == -1;
+        assert cache.get(3) == 3;
+        assert cache.get(4) == 4;
+
+        cache.put(1, 100);
+        assert cache.get(5) == -1;
+        assert cache.get(3) == 3;
+        assert cache.get(1) == 100;
+
+        //System.out.println(cache);
+    }
+
+    public static class CacheEntry {
+        private int key;
+        private int val;
+        CacheEntry next;
+        CacheEntry prev;
+
+        public CacheEntry(int key, int val){
+            this.key = key;
+            this.val = val;
+        };
+    }
+
+    public String toString() {
+        StringBuilder result = new StringBuilder();
+        CacheEntry current = mostRecently.prev;
+
+        while (current != leastRecently) {
+            result.append("[").append(current.key).append(":").append(current.val).append("] ");
+            current = current.prev;
+        }
+
+        return result.toString();
+    }
+}