【LeetCode 380】Insert Delete GetRandom O(1)

题目描述

Design a data structure that supports all following operations in average O(1) time.

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.remove(val): Removes an item val from the set if present.
3. 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();

题目大意：

设计一个数据结构，在O(1) 的时间完成上面的操作。

思路：

删除时，将最后一个元素移到被删除位置，保证index连续，随机时概率相等。

代码：

class RandomizedSet {
public:
    /** Initialize your data structure here. */
    RandomizedSet() {
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if (key2index.count(val) != 0)
            return false;
        key2index[val] = num;
        index2key[num] = val;
        num += 1;
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if (key2index.count(val) == 0)
            return false;
        int index1 = key2index[val];
        num -= 1;
        int index2 = num;
        int val2 = index2key[num];
        key2index[val2] = index1;
        index2key[index1] = val2;

        key2index.erase(val);
        index2key.erase(index2);
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        if (num == 0)
            return NULL;
        int index = (int) (rand() % (num-1)) + 1;
        return index2key[index];
    }

public:
    map<int, int>key2index;
    map<int, int>index2key;
    int num = 1;
};