LeetCode: 380-Insert Delete GetRandom O(1) 解題紀錄

Last Updated on 2022-11-29 by Clay

題目

Implement the RandomizedSet class:

• RandomizedSet() Initializes the RandomizedSet object.
• bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.
• bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.
• int getRandom() Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

You must implement the functions of the class such that each function works in average O(1) time complexity.

Example 1:

Input
["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]
Output
[null, true, false, true, 2, true, false, 2]

Explanation RandomizedSet randomizedSet = new RandomizedSet(); randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully. randomizedSet.remove(2); // Returns false as 2 does not exist in the set. randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2]. randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly. randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2]. randomizedSet.insert(2); // 2 was already in the set, so return false. randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.

Constraints:

• -231 <= val <= 231 - 1
• At most 2 * 105 calls will be made to insert, remove, and getRandom.
• There will be at least one element in the data structure when getRandom is called.

這個題目我們需要實作 insert()、remove()、getRandom() 三個 methods，而且平均時間複雜度都要求要 O(1)。

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解題思路

Python 由於語法上較為彈性，所以挺好寫的；C++ 就得分別宣告一個雜湊表跟一組整數陣列來時實作。

C++ 範例程式碼

class RandomizedSet {
private:
    vector<int> nums;
    unordered_map<int, int> setMap;
    
public:
    RandomizedSet() {
        
    }
    
    bool insert(int val) {
        // Is existed
        if (setMap.find(val) != setMap.end()) {
            return false;
        }
        
        // Store the position of `nums`
        setMap[val] = nums.size();
        nums.emplace_back(val);
        return true;
    }
    
    bool remove(int val) {
        // Is not existed
        if (setMap.find(val) == setMap.end()) {
            return false;
        }
        
        // Swap the last number and the remove number
        int lastNum = nums.back();
        setMap[lastNum] = setMap[val];
        swap(nums[nums.size()-1], nums[setMap[val]]);
        
        // Remove the target number of `nums` and `setMap`
        nums.pop_back();
        setMap.erase(val);
        
        return true;
    }
    
    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();
 */

Python 範例程式碼

class RandomizedSet:

    def __init__(self):
        self.set_map = {}        

    def insert(self, val: int) -> bool:
        is_exist = self.set_map.get(val, False)
        self.set_map[val] = True
        return not is_exist

    def remove(self, val: int) -> bool:
        is_exist = self.set_map.get(val, False)
        self.set_map.pop(val, None)
        return is_exist

    def getRandom(self) -> int:
        return random.choice(list(self.set_map.keys()))


# Your RandomizedSet object will be instantiated and called as such:
# obj = RandomizedSet()
# param_1 = obj.insert(val)
# param_2 = obj.remove(val)
# param_3 = obj.getRandom()

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References

• 380. Insert Delete GetRandom O(1)

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