Question Definition
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();
Java Solution
class RandomizedSet {
private Map<Integer, Integer> map;
private List<Integer> index;
private Random random;
private int len;
/** Initialize your data structure here. */
public RandomizedSet() {
map = new HashMap<>();
index = new ArrayList<>();
random = new Random();
len = 0;
}
/** 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, len);
index.add(len, val);
len++;
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 oldIndex = map.get(val);
len--;
map.put(index.get(len), oldIndex);
index.set(oldIndex,index.get(len));
map.remove(val);
return true;
}
/** Get a random element from the set. */
public int getRandom() {
return index.get(random.nextInt(len));
}
}
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