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Leetcode380 Insert Delete GetRandom O(1)

Leetcode380 Insert Delete GetRandom O(1)

题目描述

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:

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// 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();

思路:

Hashmap存放加入的val和在list的位置,list中存储每一个val。则每一个val对应的index为map.get(val)。如果index为null或者list.get(index)为null则代表list中没有该元素,若调用insert加入并返回true, 若调用remove则返回false。定义一个random对象,随机获取index,若list.get(index)为空,则重新随机获取。

代码

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class RandomizedSet {

HashMap<Integer,Integer> map;
List<Integer> list;
Random rd = new Random();
/** Initialize your data structure here. */
public RandomizedSet() {
map = new HashMap<>();
list = new ArrayList<>();
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
Integer index = map.get(val);
if(index != null && list.get(index)!=null )
return false;
else{
list.add(val);
map.put(val,list.size()-1);
return true;
}
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
Integer index = map.get(val);
if(index!=null&& list.get(index)!=null){
list.set(index,null);
return true;
}else
return false;

}

/** Get a random element from the set. */
public int getRandom() {
Integer index = rd.nextInt(list.size());
while(list.get(index)==null){
index = rd.nextInt(list.size());
}
return list.get(index);
}
}

/**
* 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();
*/
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