380. Insert Delete GetRandom O(1)
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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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();
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【题目分析】
设计一种数据结构，使得插入，删除和随机获取一个值的时间复杂度为O(1).
【思路】
1. 随机读取一个数的话，使用数组的时间复杂度是最小的，因为通过下标可以直接定位。考虑到元素会不断插入，我们可以选择ArrayList来存储我们的元素。
2. 要删除一个元素，在数组中需要遍历才能找到这个元素。而在HashMap中，可以快速定位一个元素。因此我们可以用HashMap来存储元素和它在ArrayList中对应的下标。
通过以上的分析，我们知道了数据结构该如何设计。
【java代码】
public class RandomizedSet {
Map<Integer, Integer> map;
List<Integer> list;
Random random;
/** Initialize your data structure here. */
public RandomizedSet() {
map = new HashMap<>();
list = new ArrayList<>();
random = new Random();
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
if(map.containsKey(val)) return false;
map.put(val, list.size());
list.add(val);
return true;
}
/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
if(!map.containsKey(val)) return false;
int loc = map.get(val);
if(loc < list.size()-1) {
int last = list.get(list.size()-1);
list.set(loc, last);
map.put(last, loc);
}
map.remove(val);
list.remove(list.size()-1);
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();
*/