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.

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.

题目大意

设计一个数据结构，实现以下三种操作：

　　1. 插入：如果插入一个已存在的数字则返回false，否则插入该数字返回true；

　　2. 删除：如果删除一个不存在的数字返回false，否则删除该数字返回true；

　　3. 获得随机数：随机获得已插入的数字其中的一个，保证所有数字都有几率获得。

示例

// 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();

解题思路

用vector<int>保存插入的整数，用map<int, int>保存存入整数的索引位置。

复杂度分析

时间复杂度：O(1)

空间复杂度：O(N)

代码

class RandomizedSet {
private:
    vector<int> nums;
    map<int, int> index;
    
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(index.find(val) != index.end())
            return false;
        // 存入整数到nums，同时记录该数字的索引位置
        nums.emplace_back(val);
        index[val] = nums.size() - 1;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(index.find(val) == index.end())
            return false;
        // 将nums的最后一个位置与val互换位置，并删除最后一个整数
        int last = nums.back();
        index[last] = index[val];
        nums[index[val]] = last;
        nums.pop_back();
        index.erase(val);
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand() % nums.size()];
    }
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet* obj = new RandomizedSet();
 * bool param_1 = obj->insert(val);
 * bool param_2 = obj->remove(val);
 * int param_3 = obj->getRandom();
 */