leetcode380. Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

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();

解法

使用一个list和一个map，list中存放元素，map中存放key：list中元素的值和value：在list中的位置。方便list进行O(1)时间复杂度的删除。

public class RandomizedSet {

List<Integer> nums;
Map<Integer, Integer> loc;
Random random;
/** Initialize your data structure here. */
public RandomizedSet() {
nums = new ArrayList<Integer>();
loc = new HashMap<Integer, Integer>();
random = new Random();
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
if (loc.containsKey(val)) {
return false;
}
nums.add(val);
loc.put(val, nums.size() - 1);
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
if (!loc.containsKey(val)) {
return false;
}
int position = loc.get(val);
if (position < nums.size() - 1) {
int lastnode = nums.get(nums.size() - 1);
nums.set(position, lastnode);
loc.put(lastnode, position);
}

nums.remove(nums.size() - 1);
loc.remove(val);
return true;
}

/** Get a random element from the set. */
public int getRandom() {
return nums.get(random.nextInt(nums.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();
*/