# 380. Insert-Delete-GetRandom-O(1)

difficulty: Medium

Design a data structure that supports all following operations in *average* O(1) time.

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1. `insert(val)`: Inserts an item val to the set if not already present.
2. `remove(val)`: Removes an item val from the set if present.
3. `getRandom`: Returns a random element from current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

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Example:

```
// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();
// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);
// Returns false as 2 does not exist in the set.
randomSet.remove(2);
// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);
// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();
// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);
// 2 was already in the set, so return false.
randomSet.insert(2);
// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();
```

## Method One

```Java
class RandomizedSet {
    private Map<Integer, Integer> cache;
    private ArrayList<Integer> array;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.cache = new HashMap<Integer, Integer>();
        this.array = new ArrayList<Integer>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if(cache.containsKey(val)) return false;
        int size = array.size();
        cache.put(val, size);
        array.add(size, val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
 // 这里的思想就是把 arralist 中最后一位和 需要被删除的那个位置调换，然后再删除最后一位。这样就避免了 O(N) 的操作，保证了O(1)
    public boolean remove(int val) {
        if(!cache.containsKey(val)) return false;
        int last = array.get(array.size() - 1);
        int index = cache.get(val);
        array.set(index, last);
        cache.put(last, index);
        array.remove(array.size() - 1);
        cache.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int index = (int) Math.floor(Math.random() * array.size() );
        return array.get(index);
    }
}
​
/**
 * 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();
 */
​
```
