380. Insert Delete GetRandom O(1)

https://leetcode.com/problems/insert-delete-getrandom-o1/?tab=Description

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(); Subscribe to see which companies asked this question.

思路

既然删除，增加，随机访问都要O（1），通过随机访问要O(1)，自然想到了数组，但是如何删除，增加都为O(1)呢？用HashMap存储位置。

代码

public class RandomizedSet { ArrayList<Integer> nums; HashMap<Integer, Integer> locs; java.util.Random rand = new java.util.Random(); /** Initialize your data structure here. */ public RandomizedSet() { nums = new ArrayList<Integer>(); locs = new HashMap<Integer, Integer>(); } /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ public boolean insert(int val) { boolean contain = locs.containsKey(val); if ( contain ) return false; locs.put( val, nums.size()); nums.add(val); return true; } /** Removes a value from the set. Returns true if the set contained the specified element. */ public boolean remove(int val) { boolean contain = locs.containsKey(val); if ( ! contain ) return false; int loc = locs.get(val); if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val int lastone = nums.get(nums.size() - 1 ); nums.set( loc , lastone ); locs.put(lastone, loc); } locs.remove(val); nums.remove(nums.size() - 1); return true; } /** Get a random element from the set. */ public int getRandom() { return nums.get( rand.nextInt(nums.size()) ); } }