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.
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| 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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Solution:
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| class RandomizedSet {
private Random RANDOM = new Random();
private Map<Integer, Integer> valueToIndex = new HashMap<>();
private List<Integer> values = new ArrayList<>();
/** Initialize your data structure here. */
public RandomizedSet() {
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
Integer index = valueToIndex.get(val);
if (index != null) {
return false;
}
values.add(val);
valueToIndex.put(val, values.size() - 1);
return true;
}
/** Removes a value from the set. Returns true
if the set contained the specified element. */
public boolean remove(int val) {
Integer index = valueToIndex.remove(val);
if (index == null) {
return false;
}
int lastIndex = values.size() - 1;
if (index != lastIndex) {
Integer prev = values.get(lastIndex);
values.set(index, prev);
valueToIndex.put(prev, index);
}
values.remove(lastIndex);
return true;
}
/** Get a random element from the set. */
public int getRandom() {
int index = RANDOM.nextInt(valueToIndex.size());
return values.get(index);
}
}
/**
* 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();
*/
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Solution 2022-01-15#
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| class RandomizedSet {
boolean debug = false;
Random random = new Random();
Map<Integer, Integer> valIndex = new HashMap<>();
Integer[] array = new Integer[2 * 10_000 + 1];
int size = 0;
public RandomizedSet() {
}
public boolean insert(int val) {
if (debug) System.out.println("i " + val);
Integer index = valIndex.get(val);
if (index != null) {
return false;
}
array[size] = val;
size++;
valIndex.put(val, size - 1);
if (debug) System.out.println("array " + array + " map " + valIndex);
return true;
}
public boolean remove(int val) {
if (debug) System.out.println("r " + val);
Integer index = valIndex.remove(val);
if (index == null) {
return false;
}
array[index] = array[size - 1];
array[size - 1] = null;
size--;
if (size != 0) {
valIndex.put(array[index], index);
}
if (debug) System.out.println("array " + array + " map " + valIndex);
return true;
}
public int getRandom() {
return array[random.nextInt(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();
*/
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