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

1 解题思想

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();
*/