Leetcode 380. Insert Delete GetRandom O(1) 常数增删改系统 解题报告

1 解题思想

这道题是要设计一个常数时间内增加，删除，随机查询的数据结构，数值不会重复。

重点在于如何做到常数内删除？

思想其实也很简单，使用一个HashMap保存每个值的位置就可以了，然后全部值存到一个ArrayList里面，删除的时候讲删除的那个位置的值进行调换，将ArrayList里面最后一位的值提到要删除的那个位置上覆盖，然后直接删除ArrayList的最后一个

对应的下一题，建议看完后再看这个：
Leetcode 381. Insert Delete GetRandom O(1) - Duplicates allowed 可重复常数增删改系统 解题报告

2 原题

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();

3 AC解

public class RandomizedSet {
    //用来索引下在ArrayList中的关系，做到快速定位
    private HashMap<Integer,Integer> locations;
    //存储原始数据
    private ArrayList<Integer> list;
    private Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.locations = new HashMap<Integer,Integer>();
        this.list = new ArrayList<Integer>();
        this.random = new Random();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    /**
     * 判断后直接插入
     * */
    public boolean insert(int val) {
        if( locations.containsKey(val) == true ) return false;
        locations.put(val,list.size());
        list.add(val);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    /**
     * 核心思想在于：把删除的元素交换到ArrayList最后一位上，这样就可以O1完成了
     * */
    public boolean remove(int val) {
        if( locations.containsKey(val) == false) return false;
        int tmpLocation = locations.get(val);
        if(tmpLocation != list.size() - 1){
            int lastVal = list.get(list.size() -1 );
            list.set(tmpLocation,lastVal);
            locations.put(lastVal,tmpLocation);
        }
        list.remove(list.size() - 1);
        locations.remove(val);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(random.nextInt(list.size()));
    }
}

/**
 * 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();
 */