[leetcode] 380. Insert Delete GetRandom O(1) 解题报告

题目链接: https://leetcode.com/problems/insert-delete-getrandom-o1/

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 thesame 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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

思路: 要在O(1)的时间内完成插入, 删除, 取随机元素. 用链表保存元素可以在O(1)内完成插入和删除, 但是无法在O(1)随机元素. 所以需要一个可以随机存取的容器. 如果使用数组的话可以在O(1)内完成插入和随机, 但是删除无法在O(1)完成, 但是如果用另外一种删除方式, 即把要删除的元素与最后的元素交换位置, 而删除最后一个元素就可以在O(1)完成. 这样的话还需要一个hash表来记录元素在数组的位置, 这样才可以来O(1)取到这个元素, 并移动到尾部. 还需要注意的是在删除一个元素的时候与另外一个元素交换了位置,
所以在hash表中还需要将另外一个元素的位置更新.

代码如下:

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(hash.count(val)) return false;
hash[val] = vec.size();
vec.push_back(val);
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if(hash.count(val)==0) return false;
int pos = hash[val];
hash[vec.back()] = pos;
swap(vec[pos], vec[vec.size()-1]);
hash.erase(val);
vec.pop_back();
return true;
}

/** Get a random element from the set. */
int getRandom() {
return vec[rand()%vec.size()];
}
private:
unordered_map<int, int> hash;
vector<int> vec;
};

/**
* 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();
*/参考: https://discuss.leetcode.com/topic/53229/c-solution-using-unordered_map-and-vector