380. Insert Delete GetRandom O(1)

题目：

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

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. Each element must have the same probability of being returned.

public class RandomizedSet {

    /** Initialize your data structure here. */
     ArrayList<Integer> array=null;
    public RandomizedSet() {
      if(array==null){
          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(array.contains(val)){
            return false;
        }else{
            array.add(val);
            return true;
        }
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
       for(int i=0;i<array.size();i++){
           if(array.get(i)==val){
               array.remove(i);
               return true;
           }
       }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        if(array==null||array.size()==0){
            return -1;
        }
        int t= (int)(Math.random()*10%array.size());
        return array.get(t);
    }
}