380. Insert Delete GetRandom O(1)

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

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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();

这题难在O(1)的复杂度

insert：

由于是hashmap，所以查找是O(1)的

remove：

在AaaryList里删除不是O（1）的，但是替换和删除最后一位是O（1）的。

所以将要删除的数替换到最后一位。

getRandom：

ArrayList里查找是O（1）的

map的value存放key在list里的位置，将要删除的数和最后一个数互换，避免了删除数组带来的时间复杂度。

public class RandomizedSet {    ArrayList<Integer> list;    HashMap<Integer, Integer> map;    java.util.Random rand = new java.util.Random();    /** Initialize your data structure here. */    public RandomizedSet() {        list = new ArrayList<Integer>();        map = 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 = map.containsKey(val);        if ( contain ) 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) {        boolean contain = map.containsKey(val);        if ( ! contain ) return false;        int index = map.get(val);        if (index < list.size() - 1 ) { // not the last one than swap the last one with this val            int lastone = list.get(list.size() - 1 );            list.set( index , lastone );            map.put(lastone, index);        }        map.remove(val);        list.remove(list.size() - 1);        return true;    }    /** Get a random element from the set. */    public int getRandom() {        return list.get( rand.nextInt(list.size()) );    }}