Insert Delete GetRandom O(1)问题及解法
2017-10-03 14:32:00      个评论    来源：刘天的博客

Insert Delete GetRandom O(1)问题及解法。问题描述：

Design a data structure that supports all following operations inaverageO(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 thesame probabilityof being returned. `

`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();``````

`问题分析：`

`利用map和vector分别保留 和 value，这里有个小知识点，搞清楚这个，此题迎刃而解。`

`如何在常量时间内删除数组的一个元素？在可改变原先顺序的基础上，我们可以将要数组最后一个元素覆盖到要删除的元素的位置，调用数组pop_back方法即可。`

`过程详见代码：`

``````class RandomizedSet {
public:
unordered_map map;
vector res;
/** Initialize your data structure here. */
RandomizedSet() {
srand(time(NULL));
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
if (!map.count(val))
{
map[val] = res.size();
res.emplace_back(val);
return true;
}
return false;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if (!map.count(val)) return false;
int t = res.size() - 1;
map[res.back()] = map[val];
res[map[val]] = res.back();
res.pop_back();
map.erase(val);
return true;
}

/** Get a random element from the set. */
int getRandom() {
int index = rand() % map.size();
return res[index];
}
};

/**
* 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();
*/``````