creating a freq map with DLL as value.

Author: Arjiit

Design/LFUcache.java

@@ -0,0 +1,109 @@
+class LFUCache {
+    class Node {
+        int key, val;
+        Node prev, next;
+        int cnt;
+        public Node(int k, int v) {
+            this.key = k;
+            this.val = v;
+            cnt = 1;
+        }
+    }
+    
+    class DLL {
+        Node head, tail;
+        int len;
+        public DLL() {
+            head = new Node(0,0);
+            tail = new Node(0, 0);
+            head.next = tail;
+            tail.prev = head;
+            len = 0; // since no node in DLL
+        }
+    
+    public void addHead(Node node) {
+        Node next = head.next; // save next address
+        head.next = node;
+        node.prev = head;
+        node.next = next;
+        next.prev = node;
+        map.put(node.key, node);
+        len++;
+    }
+    
+    public void remove(Node node) {
+        Node prev = node.prev; // save prev address
+        Node next = node.next;
+        prev.next = next;
+        next.prev = prev;
+        map.remove(node.key);
+        len--;
+    }
+    
+    public void removeTail() {
+        Node prevTail = tail.prev;
+        remove(prevTail);
+    }
+    }
+    Map<Integer, DLL> freq;
+    Map<Integer, Node> map; // for (key, node) pair
+    int capacity, size;
+    int maxFreq;
+    public LFUCache(int capacity) {
+        freq = new HashMap<>();
+        map = new HashMap<>();
+        this.capacity = capacity;
+        size = 0;
+        maxFreq = 0;
+    }
+    
+    public int get(int key) {
+        if (map.get(key) == null) return -1;
+        Node node = map.get(key);
+        int prevFreq = node.cnt;
+        DLL prevList = freq.get(prevFreq);
+        prevList.remove(node);
+        int curFreq = prevFreq + 1;
+        maxFreq = Math.max(maxFreq, curFreq);
+        DLL curList = freq.getOrDefault(curFreq, new DLL()); // since there may not be a DLL on that frequency
+        node.cnt++;
+        curList.addHead(node);
+        freq.put(prevFreq, prevList); // updating prevfreq list and curfreq list
+        freq.put(curFreq, curList);
+        return node.val;
+    }
+    
+    public void put(int key, int value) {
+        if (capacity == 0) return; // we can't put
+        if (map.get(key) != null) { // meaning a node exists
+            map.get(key).val = value; //update its value
+            get(key); // since we are accessing it, we have to increase it's freq
+            return;
+        }
+        Node newNode = new Node(key, value);
+        DLL curList = freq.getOrDefault(1, new DLL()); // get the DLL corresponding to freq 1 or create a new one
+        curList.addHead(newNode);
+        size++;
+        if (size > capacity) {
+            if (curList.len > 1) { // if freq 1 list has nodes in it, remove last one
+                curList.removeTail();
+            } else {
+                for (int i=2; i<=maxFreq; i++) {
+                    if (freq.get(i) != null && freq.get(i).len > 0) {
+                        freq.get(i).removeTail();
+                        break;
+                    }
+                }
+            }
+            size--;
+        }
+        freq.put(1, curList);
+    }
+}
+
+/**
+ * Your LFUCache object will be instantiated and called as such:
+ * LFUCache obj = new LFUCache(capacity);
+ * int param_1 = obj.get(key);
+ * obj.put(key,value);
+ */