0380._Insert_Delete_GetRandom_O(1).md

380. Insert Delete GetRandom O(1)

难度: Medium

刷题内容

原题连接

• https://leetcode.com/problems/insert-delete-getrandom-o1/description/

内容描述

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.
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();

解题方案

思路 1 - 时间复杂度: O(1)- 空间复杂度: O(N)******

因为对于插入，删除还有同可能性返回数都要O(1)，我们知道以下几种典型的数据结构

dictionary
list
set
LinkedList

想要删除确定数字必须要知道数字的index，所以list肯定需要，然后怎么通过O(1)时间得到要删除元素的index呢？

mock的时候我没有想出来，墨汁大佬给了hint才想出来的

然后我就想到用字典，key,value分别是element和其index

然后想要O(1)时间同可能性返回数都要，必须要知道总共有多少个数字，那么就要维护一个self.length才行

beats 98.71%

import random
class RandomizedSet(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.cache = {}
        self.lst = []
        

    def insert(self, val):
        """
        Inserts a value to the set. Returns true if the set did not already contain the specified element.
        :type val: int
        :rtype: bool
        """
        if val not in self.cache:
            self.lst.append(val)
            self.cache[val] = len(self.lst) - 1
            return True
        return False
        

    def remove(self, val):
        """
        Removes a value from the set. Returns true if the set contained the specified element.
        :type val: int
        :rtype: bool
        """
        if val in self.cache:
            idx = self.cache[val]
            self.lst[idx] = self.lst[-1]  # 用最后一个元素替换，避免数组元素前移
            self.cache[self.lst[idx]] = idx  # 更新最后一个元素的下标值 
            self.lst.pop()
            del self.cache[val]
            return True
        return False

        
    def getRandom(self):
        """
        Get a random element from the set.
        :rtype: int
        """
        # 这种是常规方法
        # idx = random.randint(0, self.length-1)
        # return self.lst[idx]
        # 神奇函数
        return random.choice(self.lst)

思路 2

增删时间复杂度为 O(1)，这就是字典的特征

class RandomizedSet:   
    def __init__(self):
        self.data = dict()

    def insert(self, val: int) -> bool:
        if val in self.data:
            return False
        else: 
            self.data[val] = val
            return True

    def remove(self, val: int) -> bool:
        if val in self.data:
            del self.data[val]
            return True
        else:
            return False

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