## 题目描述：

LeetCode 380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in O(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 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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();
```

## 题目大意：

insert(val): 如果集合中不包含val，则插入val

remove(val): 如果集合中包含val，则移除val

getRandom: 从现有集合中随机返回一个元素，每个元素被返回的概率应该相等

## 解题思路：

```insert(val): 将val添至dataList末尾，并在dataMap中保存val的下标idx

remove(val): 记val的下标为idx，dataList末尾元素为tail，弹出tail并将其替换至idx处，在dataMap中更新tail的下标为idx，最后从dataMap中移除val

getRandom: 从dataList中随机选取元素返回```

## Python代码：

``````import random
class RandomizedSet(object):

def __init__(self):
"""
"""
self.dataMap = {}
self.dataList = []

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 in self.dataMap:
return False
self.dataMap[val] = len(self.dataList)
self.dataList.append(val)
return True

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 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):
"""
Get a random element from the set.
:rtype: int
"""
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()
``````

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1. zy 发布于 2016年8月7日 22:31 #

原来还有leetcode这东西，受教了。我肯定做不出这些题。如果，我没记错大学时候，数据结构这门课，只得60分，应该是老师，放水。