leetcode380. Insert Delete GetRandom O(1)

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本文介绍了一种能在平均O(1)时间内完成插入、删除和随机获取操作的数据结构。通过结合使用列表和映射的方式，确保了在进行删除操作时也能保持高效。同时提供了完整的Java代码实现。

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();
 */