插入、删除和随机查询时间复杂度都为O(1) leetcode 381

插入、删除和随机查询时间复杂度都为O(1)

解题思路：map+list

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;

class RandomizedCollection {
    public static void main(String[] args) {
        RandomizedCollection s = new RandomizedCollection();
        s.insert(1);
        s.remove(1);
        s.insert(1);
        System.out.println(s.list);
        
    }
    public Map<Integer,List<Integer>> map = new HashMap<>();
    public List<Integer> list = new ArrayList<>();
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        
    }
    
    /** 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);
        List<Integer> temp = map.get(val);
        if(temp==null){
            temp = new ArrayList<>();
            temp.add(list.size()-1);
            map.put(val,temp);
            return true;
        }else{
            temp.add(list.size()-1);
            return false;
        }
        
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        List<Integer> temp = map.get(val);
        if(temp==null){
            return false;
        }
        int item = temp.remove(0);
        if(temp.size()==0) {
            map.remove(val);
        }
        if(list.size()-1==item) {
            list.remove(item);
        }else {
            Integer lastItem = list.remove(list.size()-1);//最后一个元素的内容
            list.set(item, lastItem);
            //对结尾元素的调整
            temp = map.get(lastItem);
            temp.remove((Integer)list.size());//这个过程还是比较耗时的
            temp.add(item);
        }
        return true; 
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        Random random = new Random();
        int o = random.nextInt(list.size());
        return list.get(o);
    }
}