# Python code cheatsheet **Random** `import random`\ `random_idx = random.randint(low, high)` Default Min and Max * Use `float('inf')` for positive infinity. * Use `float('-inf')` for negative infinity.xfcha dict ```python nodeInfo = dict() nodeInfo[ng] = nodeInfo.get(ng, 0) + 1 #有则返回vaL 无则返回0 这样可以无需初始化 ``` **如何统计字典中某个值的出现次数？** 用 `list(dict.values()).count(x)` #### 🔍 `parent.get(key)` vs `parent[key]` * `parent[key]`：如果字典中没有这个 key，会直接抛出 **`KeyError`** * `parent.get(key)`：如果 key 不存在，不会报错，而是返回 **`None`** deque 一般 qfs里使用， 用popleft取代list.pop(0), time complexity: o(n) => o(1) ``` q = deque() cur = q.popleft() ``` 数组slicing reverse ： s\[::-1] start:stop:step ``` s = "hello" reversed_s = s[::-1] print(reversed_s) # Output: "olleh" ``` ``` presums[-1] will return last element of the list ``` 二维数组 ``` dp = [[0]*(m+1) for _ in range(n+1)] ``` ``` n//2 vs n/2 n//2 floor, return int n/2 true division, return float ``` Ord(): return unicode code point of a charactor ``` print(ord('a')) # Output: 97 ``` sort 自定义函数 ``` pairs.sort(key = lambda x: (x[1],x[0]))#记得要加括号！！！！ ``` \ \*list will unpack the list ``` workers = [1, 2, 3, 4] # Using *workers in a print statement print(*workers) # Output: 1 2 3 4 ``` default: **Functions**: `max()` and `min()` support the `default` parameter. 
min/max([], default=0)  # Returns 0
遍历dictionary: ```` ```python for k,v in d.items(): v.reserse() #v is a list ``` ```` number char coversion `char.isdigit()` `char.isalpha()` #### **Convert a Character to its ASCII/Unicode Number** `char = 'A' number = ord(char) print(number) # Output: 65` #### **Convert a Numeric Character to its Integer Value** `char = '7' number = int(char) print(number) # Output: 7` **Counter** `Counter(list)` returnr dict. **most\_common**(k) returns a list of `(element, count)` tuples sorted by counts. ``` return [i for i,_ in collections.Counter(nums).most_common(k)] ``` **Sort** Define a lambda function to sort various data structures like lists, dictionaries, and tuples. **In-place vs. Copy**: * `sorted()` returns a new sorted list. * `.sort()` sorts the list in place and returns `None`. #### Sorting a List ``` # Example list of tuples data = [(1, 3), (4, 1), (5, 2)] # Sorting the list based on the second element of each tuple sorted_data = sorted(data, key=lambda x: x[1]) # Output: [(4, 1), (5, 2), (1, 3)] print(sorted_data) ``` #### Sorting a Dictionary by Value ``` # Example dictionary data = {'a': 3, 'b': 1, 'c': 2} # Sorting the dictionary by its values sorted_data = sorted(data.items(), key=lambda x: x[1]) # Output: [('b', 1), ('c', 2), ('a', 3)] print(sorted_data) ``` #### Sorting a List of Dictionaries ``` # Example list of dictionaries data = [{'name': 'John', 'age': 25}, {'name': 'Jane', 'age': 22}, {'name': 'Doe', 'age': 30}] # Sorting by the 'age' key sorted_data = sorted(data, key=lambda x: x['age']) # Output: [{'name': 'Jane', 'age': 22}, {'name': 'John', 'age': 25}, {'name': 'Doe', 'age': 30}] print(sorted_data) ``` deque 整个list 加入deque, extend() ``` d.extend(lst) ``` use cache in DP ``` from functools import lru_cache # Import lru_cache @lru_cache(None) ``` filter 参数是一个函数和iterator，函数作用在每个iterator上 
#滤掉所有非(),因为count返回返回1或0
s = "a(b)c)d"
result = ''.join(filter('()'.count, s))
print(result)  # Output: "()()"
**Concise code** **1️⃣ Multiple Appends in One Line** Instead of: ```python pythonCopyEditqueue.append(item1) queue.append(item2) ``` Use: ```python pythonCopyEditqueue.extend([item1, item2]) ``` Or: ```python pythonCopyEditqueue += item1, item2 # No need for extra brackets! ``` *** **2️⃣ Inline If-Else (`or` Trick)** Instead of: ```python pythonCopyEditif name: greeting = name else: greeting = "Guest" ``` Use: ```python pythonCopyEditgreeting = name or "Guest" ``` * If `name` is `None` or `""`, it defaults to `"Guest"`. *** **3️⃣ Swapping Variables Without Temp** Instead of: ```python pythonCopyEdittemp = a a = b b = temp ``` Use: ```python pythonCopyEdita, b = b, a ``` *** **4️⃣ List Comprehension Instead of Loops** Instead of: ```python pythonCopyEditsquares = [] for i in range(10): squares.append(i * i) ``` Use: ```python pythonCopyEditsquares = [i * i for i in range(10)] ``` *** **5️⃣ Using `defaultdict` to Avoid Key Errors** Instead of: ```python pythonCopyEditword_count = {} for word in words: if word in word_count: word_count[word] += 1 else: word_count[word] = 1 ``` Use: ```python pythonCopyEditfrom collections import defaultdict word_count = defaultdict(int) for word in words: word_count[word] += 1 ``` *** **6️⃣ Merging Two Dictionaries** Instead of: ```python pythonCopyEditdict1 = {"a": 1, "b": 2} dict2 = {"b": 3, "c": 4} merged = dict1.copy() merged.update(dict2) ``` Use (Python 3.9+): ```python pythonCopyEditmerged = dict1 | dict2 ``` *** **7️⃣ Unpacking Iterables** Instead of: ```python pythonCopyEdita = my_list[0] b = my_list[1] c = my_list[2:] ``` Use: ```python pythonCopyEdita, b, *c = my_list ``` *** **BIT** | `bin(mask)` | 转二进制字符串（带 `0b` 前缀） | `'0b101'` | | ----------- | ------------------ | --------- | | `mask.bit_count()` | 直接统计 `1` 的个数（Python 3.10+） | `2`（若 `mask = 0b1010`） | | ------------------ | -------------------------- | ---------------------- | #### **`pos = available & -available` 的作用：取最低位的 `1`** **核心原理** * **`available`**：一个二进制数，表示当前可选的位置（`1` 表示可选，`0` 表示不可选）。 * 例如 `available = 0b1010` 表示第 2 列和第 4 列可选（从右往左数，最低位是第 1 列）。 * **`-available`**：计算机中负数以补码表示，`-x = ~x + 1`。 * 例如 `available = 0b1010`，则 `-available = ~0b1010 + 1 = 0b0101 + 1 = 0b0110`。 * **`available & -available`**： * 保留 `available` 最低位的 `1`，其余位全置 `0`。 * 例如 `0b1010 & 0b0110 = 0b0010`（保留了最低位的 `1`）。 **Count** | **数据类型** | **是否支持 `.count()`** | **统计对象** | **示例** | | ----------- | ------------------- | -------- | --------------------------- | | 字符串（`str`） | ✅ | 子字符串 | `"hello".count("l")` | | 列表（`list`） | ✅ | 元素 | `[1, 2, 2].count(2)` | | 元组（`tuple`） | ✅ | 元素 | `(1, 2, 2).count(2)` | | 字节（`bytes`） | ✅ | 子字节序列 | `b"hello".count(b"l")` | | 集合（`set`） | ❌ | 无 | 需手动遍历 | | 字典（`dict`） | ❌ | 无 | `list(d.values()).count(1)` | | 数字（`int`） | ❌ | 无 | 不适用 | **Optional**\ `Optional[Type]` 表示变量可以是 `Type` 类型或 `None` `Optional[Type] = Union[Type, None] = Type|None`