add: LRU缓存机制

src/array/lru-cache.js

@@ -0,0 +1,21 @@
+export default class LRUCache {
+  constructor (capacity) {
+    this.max = capacity; this.map = new Map()
+  }
+
+  get (key) {
+    const value = this.map.get(key)
+    if (value !== undefined) {
+      this.map.delete(key)
+      this.map.set(key, value)
+      return value
+    }
+    return -1
+  }
+
+  put (key, value) {
+    this.map.delete(key)
+    this.map.set(key, value)
+    if (this.map.size > this.max) this.map.delete(this.map.keys().next().value)
+  }
+}