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();
class RandomizedSet { List<Integer> set; /** Initialize your data structure here. */ public RandomizedSet() { set = 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) { if(set.contains(val)) return false; else{ set.add(new Integer(val)); return true; } } /** Removes a value from the set. Returns true if the set contained the specified element. */ public boolean remove(int val) { if(set.contains(val)) { set.remove(new Integer(val)); return true; } else return false; } /** Get a random element from the set. */ public int getRandom() { Random r = new Random(); return set.get(r.nextInt(set.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(); */
干？
class RandomizedSet { Map<Integer, Integer> map = new HashMap<>(); List<Integer> list = new ArrayList<>(); Random rnd = new Random(); public boolean insert(int val) { if(map.containsKey(val)) { return false; } map.put(val, list.size()); list.add(val); return true; } public boolean remove(int val) { if(!map.containsKey(val)) { return false; } // Index of value(key) in map int idx = map.remove(val); //The last element. int last = list.remove(list.size() - 1); //If equal, means the last one accidently equals to what‘s going to be deleted. if(val != last) { //If not, reset the last element to the list as index = index of what‘s been deleted. list.set(idx, last); //Update the index of that element. map.put(last, idx); } return true; } public int getRandom() { return list.get(rnd.nextInt(list.size())); } }
380. Insert Delete GetRandom O(1)
原文：https://www.cnblogs.com/wentiliangkaihua/p/11865586.html