标签：put anti etc des return led lan let href
原题链接在这里：https://leetcode.com/problems/insert-delete-getrandom-o1/?tab=Description
题目：
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();
题解：
用list保存insert进来的数字. 利用HashMap<Integer, Integer> hm保存insert进来数字和对应的index.
insert, remove时对应更改list, hm.
getRandom在list上用random index get一个数.
Time Complexity: inset, O(1). remove, O(1). getRandom, O(1).
Space: O(n).
AC Java:
1 public class RandomizedSet { 2 List<Integer> ls; 3 HashMap<Integer, Integer> hm; 4 Random rand; 5 6 /** Initialize your data structure here. */ 7 public RandomizedSet() { 8 ls = new ArrayList<Integer>(); 9 hm = new HashMap<Integer, Integer>(); 10 rand = 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(hm.containsKey(val)){ 16 return false; 17 } 18 ls.add(val); 19 hm.put(val, ls.size()-1); 20 return true; 21 } 22 23 /** Removes a value from the set. Returns true if the set contained the specified element. */ 24 public boolean remove(int val) { 25 if(!hm.containsKey(val)){ 26 return false; 27 } 28 29 int ind = hm.get(val); 30 //swap当前数字和ls的最后一个数字，方便在list上remove当前数字时不影响其他数字index 31 if(ind < ls.size()-1){ 32 int lastElement = ls.get(ls.size()-1); 33 ls.set(ind, lastElement); 34 hm.put(lastElement, ind); 35 } 36 37 hm.remove(val); 38 ls.remove(ls.size()-1); 39 return true; 40 } 41 42 /** Get a random element from the set. */ 43 public int getRandom() { 44 return ls.get(rand.nextInt(ls.size())); 45 } 46 } 47 48 /** 49 * Your RandomizedSet object will be instantiated and called as such: 50 * RandomizedSet obj = new RandomizedSet(); 51 * boolean param_1 = obj.insert(val); 52 * boolean param_2 = obj.remove(val); 53 * int param_3 = obj.getRandom(); 54 */
跟上Insert Delete GetRandom O(1) - Duplicates allowed.
LeetCode Insert Delete GetRandom O(1)
标签：put anti etc des return led lan let href
原文地址：http://www.cnblogs.com/Dylan-Java-NYC/p/6439780.html
