标签：drop 快速定位 leetcode ext blog ash 数据结构 而在 following
Description Submission Solutions
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】
1 public class RandomizedSet { 2 Map<Integer, Integer> map; 3 List<Integer> list; 4 Random random; 5 6 /** Initialize your data structure here. */ 7 public RandomizedSet() { 8 map = new HashMap<>(); 9 list = new ArrayList<>(); 10 random = new Random(); 11 } 12 13 /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ 14 public boolean insert(int val) { 15 if(map.containsKey(val)) return false; 16 map.put(val, list.size()); 17 list.add(val); 18 return true; 19 } 20 21 /** Removes a value from the set. Returns true if the set contained the specified element. */ 22 public boolean remove(int val) { 23 if(!map.containsKey(val)) return false; 24 int loc = map.get(val); 25 if(loc < list.size()-1) { 26 int last = list.get(list.size()-1); 27 list.set(loc, last); 28 map.put(last, loc); 29 } 30 map.remove(val); 31 list.remove(list.size()-1); 32 return true; 33 } 34 35 /** Get a random element from the set. */ 36 public int getRandom() { 37 return list.get(random.nextInt(list.size())); 38 } 39 } 40 41 /** 42 * Your RandomizedSet object will be instantiated and called as such: 43 * RandomizedSet obj = new RandomizedSet(); 44 * boolean param_1 = obj.insert(val); 45 * boolean param_2 = obj.remove(val); 46 * int param_3 = obj.getRandom(); 47 */
LeetCode 380. Insert Delete GetRandom O(1)
标签：drop 快速定位 leetcode ext blog ash 数据结构 而在 following
原文地址：http://www.cnblogs.com/liujinhong/p/6442691.html
