380. Insert Delete GetRandom O(1)

题目分析

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.
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();

代码

class RandomizedSet {
    
    HashMap map;
    ArrayList list;
    Random rmd;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap();
        list = new ArrayList();
        rmd = new Random();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if(map.containsKey(val)) return false;
        map.put(val, list.size());
        list.add(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(!map.containsKey(val)) return false;
        // 返回的是移除元素在 ArrayList 中的位置
        int index = map.remove(val);
        // 返回的是最后一个元素的值
        int lastVal = list.remove(list.size() - 1);
        // 要移除的元素不是最后一个
        if(index != list.size()) {
            list.set(index, lastVal);
            map.put(lastVal, index);
        }
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rmd.nextInt(list.size()));
    }
}

/**
 * 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();
 */