380. Insert Delete GetRandom O(1)

java/380-Insert-Delete-GetRandom-O(1).java

@@ -0,0 +1,57 @@
+class RandomizedSet {
+    private Map<Integer, Integer> indexing;
+    private List<Integer> numbers;
+
+    public RandomizedSet() {
+        this.indexing = new HashMap<>();
+        this.numbers = new ArrayList<>();
+    }
+
+    // Append to the end, maintain indexing
+    // Delete by swapping with the end, maintain indexing
+    // Get random by getting a random index wrt list's size
+
+    public boolean insert(int val) {
+        if(this.indexing.containsKey(val)) {
+            return false;
+        }
+        int indexInsert = this.numbers.size();
+        this.numbers.add(val);
+        this.indexing.put(val, indexInsert);
+        return true;
+    }
+
+    public boolean remove(int val) {
+        if(!this.indexing.containsKey(val)) {
+            return false;
+        }
+
+        int lastIndex = this.numbers.size() - 1;
+        int lastElement = this.numbers.get(lastIndex);
+        int indexElement = this.indexing.get(val);
+
+        // Swap with last element
+        this.numbers.set(indexElement, lastElement);
+
+        // Update indices [Add & Delete]
+        this.indexing.put(lastElement, indexElement);
+        this.indexing.remove(val);
+
+        // Remove from list
+        this.numbers.remove(lastIndex);
+        return true;
+    }
+
+    public int getRandom() {
+        int randomIndex = (int) (Math.random() * this.numbers.size());
+        return this.numbers.get(randomIndex);
+    }
+}
+
+/**
+ * Your RandomizedSet object will be instantiated and called as such:
+ * RandomizedSet obj = new RandomizedSet();
+ * boolean param_1 = obj.insert(val);
+ * boolean param_2 = obj.remove(val);
+ * int param_3 = obj.getRandom();
+ */