lintcode(657)Insert Delete GetRandom O(1)

描述：

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

• insert(val): Inserts an item val to the set if not already present.
• remove(val): Removes an item val from the set if present.
• getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

样例：

// 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();

思路：

建立hashset用来查询，建立arraylist用于存储，删除时注意将末尾元素调整到空缺位置

public class RandomizedSet {
    private HashMap<Integer , Integer> store = new HashMap<Integer , Integer>();
    private ArrayList<Integer> record = new ArrayList<Integer>();
    public RandomizedSet() {
        // do initialize if necessary
    }
    
    // Inserts a value to the set
    // Returns true if the set did not already contain the specified element or false
    public boolean insert(int val) {
        // Write your code here
        if(!store.containsKey(val)){
            int num = record.size();
            store.put(val , num);
            record.add(val);
            return true;
        }
        return false;
    }
    
    // Removes a value from the set
    // Return true if the set contained the specified element or false
    public boolean remove(int val) {
        // Write your code here
        if(store != null && store.containsKey(val)){
            int num = store.get(val);
            if(num < record.size() - 1){
                record.set(num , record.get(record.size() - 1));    
            }
            record.remove(record.size() - 1);
            return true;
        }
        return false;
    }
    
    // Get a random element from the set
    public int getRandom() {
        // Write your code here
        int len = record.size() - 1;
        int result = (int)(Math.random()*len);
        return record.get(result);
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param = obj.insert(val);
 * boolean param = obj.remove(val);
 * int param = obj.getRandom();
 */