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(); */