Leetcode SQL

380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

  1. : Inserts an item val to the set if not already present.
  2. : Removes an item val from the set if present.
  3. : Returns a random element from 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.

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();

Logic: Follow the function calls the question sets for us.

A few thins to pay attention:

  1. create self.dataMap and self.dataList. Always use them with “self.” preceding
  2. On the third step, swap the idx and the tail, if idx < len(dataList)
  3. The syntax for idx and tail are idx = self.dataMap[val]; tail = self.dataList.pop()

Solution:

class RandomizedSet:
def __init__(self):
"""
Initialize your data structure here.
"""
self.dataMap = {}
self.dataList = []

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

def remove(self, val: int) -> bool:
"""
Removes a value from the set. Returns true if the set contained the specified element.
"""
if val not in self.dataMap:
return False
idx = self.dataMap[val]
tail = self.dataList.pop()
if idx < len(self.dataList):
self.dataList[idx] = tail
self.dataMap[tail] = idx
del self.dataMap[val]
return True
def getRandom(self) -> int:
"""
Get a random element from the set.
"""
return random.choice(self.dataList)
# 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()

Link

--

--

My homepage to record my thought processes for solving SQL and Algorithm questions

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store