p380

LeetCode p380 Insert Delete GetRandom O(1) 题解

1.题目：

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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

题意：

实现以上这个数据结构的，添加，删除，随机取值。

2.解题思路:

见代码。&&使用ArrayList在随机获取的时候会快一些。使用LinkedList则会超时。

3.代码

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public class RandomizedSet {

    private HashSet<Integer> set;
	private List<Integer> list;
	Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        set = new HashSet<Integer>();
		list = new ArrayList<Integer>();
		random = 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 (set.contains(val)) {
			return false;

		} else {
			set.add(val);
			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 (set.contains(val)) {
			set.remove(val);
			return true;
		} else {
			return false;
		}

    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int size = list.size();
		int a = random.nextInt(size);//生成  [0, size) 的随机数 
		while (!set.contains(list.get(a)))
		{
			a = random.nextInt(size);
		}
		return list.get(a);
    }
}

/**
 * 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();
 */

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4.一些总结：