[LeetCode] Insert Delete GetRandom O(1)

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

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.RandomizedCollection collection = new RandomizedCollection();// Inserts 1 to the collection. Returns true as the collection did not contain 1.collection.insert(1);// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].collection.insert(1);// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].collection.insert(2);// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.collection.getRandom();// Removes 1 from the collection, returns true. Collection now contains [1,2].collection.remove(1);// getRandom should return 1 and 2 both equally likely.collection.getRandom();

因为需要o(1) 时间复杂度实现getRandom() 所以需要ArrayList 另外insert和remove也要是o(1)，所以需要HashMap

代码如下:

public class RandomizedCollection {//首先  ArrayList删除最后一个元素的时间复杂度是o(1) 具体参见jdk源码 ArrayList<Integer> list; HashMap<Integer,ArrayList<Integer>> map; Random ran=new Random();    /** Initialize your data structure here. */    public RandomizedCollection() {        list=new ArrayList<Integer>();        map=new HashMap<Integer,ArrayList<Integer>>();    }        /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */    public boolean insert(int val) {        list.add(val);        if(!map.containsKey(val)){        map.put(val,new ArrayList<Integer>());        }        map.get(val).add(list.size()-1);        return true;    }        /** Removes a value from the collection. Returns true if the collection contained the specified element. */    public boolean remove(int val) {        if(!map.containsKey(val)) return false;        ArrayList<Integer> mapList=map.get(val);        int pos=mapList.remove(mapList.size()-1);        if(mapList.isEmpty()) map.remove(val);        if(pos!=list.size()-1){        Integer temp=list.get(list.size()-1);        list.set(pos,temp);        map.get(temp).remove(new Integer(list.size()-1));        map.get(temp).add(pos);        }        list.remove(list.size()-1);        return true;    }        /** Get a random element from the collection. */    public int getRandom() {        return list.get(ran.nextInt(list.size()));    }}/** * Your RandomizedCollection object will be instantiated and called as such: * RandomizedCollection obj = new RandomizedCollection(); * boolean param_1 = obj.insert(val); * boolean param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */

之前写的代码不不符合条件的，getRandom不是o(1)。

public class RandomizedCollection {Random random=new Random();HashMap<Integer,Integer> map;int size;    /** Initialize your data structure here. */    public RandomizedCollection() {        map=new HashMap<Integer,Integer>();        size=0;    }        /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */    public boolean insert(int val) {    size++;        if(!map.containsKey(val)){        map.put(val, 1);        return true;        }else{        map.put(val, map.get(val)+1);        return false;        }    }    /** Removes a value from the collection. Returns true if the collection contained the specified element. */    public boolean remove(int val) {        if(!map.containsKey(val)) return false;        else{        Integer a=map.get(val);        if(a<=1) map.remove(val);        else{        map.put(val, a-1);        }        size--;        }return true;            }    /** Get a random element from the collection. */    public int getRandom() {        int ran=random.nextInt(size);        int i=-1;        for(Integer a:map.keySet()){        int times=map.get(a);        if(ran>i&&ran<=i+times){        return a;        }        i+=times;        }        return -1;    }}/** * Your RandomizedCollection object will be instantiated and called as such: * RandomizedCollection obj = new RandomizedCollection(); * boolean param_1 = obj.insert(val); * boolean param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */