380. Insert Delete GetRandom O(1)

标签：Insert set return val nums GetRandom 380 randomSet true

380. Insert Delete GetRandom O(1)

标签（空格分隔）： leetcode array medium

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题目

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.
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();

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思路

这类问题一开始会想到用set来解决，但是在返回随机数值时，由于set不支持[]操作符，所以没法达到O(1)时间。于是把set改成unordered_map,后来同样在做返回随机数值时遇到问题，因为随机返回数组下标，map没法直接输出。因此额外维护一个vector数组用来存储数值下标。但是vector数组在进行删除操作时需要注意删除的方法：
首先根据map找到要删除元素val的下表index,然后在vector将nums[index]与末尾数组互换，最后使用pop方法删除，这样能保证O(1)的时间。

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 (m.find(val) != m.end()) return false;
        nums.emplace_back(val);
        m[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 (m.find(val) == m.end()) return false;
        int last = nums.back();
        m[last] = m[val];
        nums[m[val]] = last;
        nums.pop_back();
        m.erase(val);
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand() % nums.size()];
    }
private:
    vector<int> nums;
    unordered_map<int, int> m;
};

标签：Insert,set,return,val,nums,GetRandom,380,randomSet,true
From： https://blog.51cto.com/u_15996214/6105919