altruistcoder · modestlearner · Oct 17, 2021
diff --git a/Queue/lru.cpp b/Queue/lru.cpp
@@ -0,0 +1,87 @@
+// We can use stl container list as a double
+// ended queue to store the cache keys, with
+// the descending time of reference from front
+// to back and a set container to check presence
+// of a key. But to fetch the address of the key
+// in the list using find(), it takes O(N) time.
+// This can be optimized by storing a reference
+//	 (iterator) to each key in a hash map.
+#include <bits/stdc++.h>
+using namespace std;
+
+class LRUCache {
+	// store keys of cache
+	list<int> dq;
+
+	// store references of key in cache
+	unordered_map<int, list<int>::iterator> ma;
+	int csize; // maximum capacity of cache
+
+public:
+	LRUCache(int);
+	void refer(int);
+	void display();
+};
+
+// Declare the size
+LRUCache::LRUCache(int n)
+{
+	csize = n;
+}
+
+// Refers key x with in the LRU cache
+void LRUCache::refer(int x)
+{
+	// not present in cache
+	if (ma.find(x) == ma.end()) {
+		// cache is full
+		if (dq.size() == csize) {
+			// delete least recently used element
+			int last = dq.back();
+
+			// Pops the last element
+			dq.pop_back();
+
+			// Erase the last
+			ma.erase(last);
+		}
+	}
+
+	// present in cache
+	else
+		dq.erase(ma[x]);
+
+	// update reference
+	dq.push_front(x);
+	ma[x] = dq.begin();
+}
+
+// Function to display contents of cache
+void LRUCache::display()
+{
+
+	// Iterate in the deque and print
+	// all the elements in it
+	for (auto it = dq.begin(); it != dq.end();
+		it++)
+		cout << (*it) << " ";
+
+	cout << endl;
+}
+
+// Driver Code
+int main()
+{
+	LRUCache ca(4);
+
+	ca.refer(1);
+	ca.refer(2);
+	ca.refer(3);
+	ca.refer(1);
+	ca.refer(4);
+	ca.refer(5);
+	ca.display();
+
+	return 0;
+}
+// This code is contributed by Satish Srinivas