HashTable-380-Insert Delete GetRandom O(1)
2018-01-24 19:35
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Description:
Design a data structure that supports all following operations in average O(1) time.
Example:
Solution:
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();
Solution:
public class RandomizedSet { ArrayList<Integer> nums; HashMap<Integer, Integer> locs; java.util.Random rand = new java.util.Random(); /** Initialize your data structure here. */ public RandomizedSet() { nums = new ArrayList<Integer>(); locs = new HashMap<Integer, Integer>(); } /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ public boolean insert(int val) { boolean contain = locs.containsKey(val); if ( contain ) return false; locs.put( val, nums.size()); nums.add(val); return true; } /** Removes a value from the set. Returns true if the set contained the specified element. */ public boolean remove(int val) { boolean contain = locs.containsKey(val); if ( ! contain ) return false; int loc = locs.get(val); if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val int lastone = nums.get(nums.size() - 1 ); nums.set( loc , lastone ); locs.put(lastone, loc); } locs.remove(val); nums.remove(nums.size() - 1); return true; } /** Get a random element from the set. */ public int getRandom() { return nums.get( rand.nextInt(nums.size()) ); } }
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