9 return s == s[::-1] # "".join(list(reversed(s)))
28 haystack.find(needle) # cf. index() return error if not found, but find() returns -1
191 return str(bin(n))[2:].count('1')
# strip, replace, upper, lower, startswith, endswith etc..70 memo = [-1]*(n+1)
215 l = [-x for x in nums]
67 return "".join(l[::-1])
58 return len(s.strip().split()[-1])
189 nums[:] = nums[-k:] + nums[:-k]
189 nums[:k], nums[k:] = nums[-k:], nums[:-k]
189 val = nums.pop() # O(1)
189 nums.insert(0, val) # O(n)
463 adj = [(row,col-1), (row,col+1), (row-1,col), (row+1,col)]
463 adj_land = [(r,c) for r,c in adj if 0 <= r < m and 0 <= c < n and grid[r][c]==1]
1051 expected.extend([i]*counter[i])
6 l = [[0 for _ in range(n)] for _ in range(n)] # Do not initialize l = [[None] * n] * n
78 l_subsets.append(tmp.copy()) # Do not append list tmp itself
1905 grid = copy.deepcopy(grid2) # must use deep copy for 2D array
75 nums.clear() # nums[:] = []
from functools import reduce
136 return reduce(lambda x, y: x ^ y, nums, 0)
90 return list(map(list, ret)) # ret={(1, 2), (2,), (1,), (1, 2, 2), (2, 2), ()}
72 import numpy as np
72 print(np.array(cost))
[[0 1 2 3 4 5]
[1 0 0 0 0 0]
[2 0 0 0 0 0]
[3 0 0 0 0 0]]# append(), pop()
67 l1 = [int(x) for x in a]
67 n1 = l1.pop() if l1 else 0 # Add Binary
67 n2 = l2.pop() if l2 else 0
1971 stack = [source]
1971 while stack:
1971 vertex = stack.pop() # dfs
1971 stack.append(adj)
77 tmp.append(num)
77 dfs(num + 1)
77 tmp.pop() # backtracking# append(), popleft()
637 from collections import deque
637 q = deque([root])
637 node = q.popleft() # bfs
637 q.append(node.left)
67 q.appendleft(carry)
67 return "".join(map(str, q))
# fyi.Deque means double-ended queue, works as a queue and a stack at the same time.
# But, slicing such as [1:] is not supported, index access is O(n)
71 from collections import deque
71 path_split = deque([])
71 path_split.pop() # stack
71 path_split.popleft() # queue
71 return "/" + "/".join(path_split)# del d[key], val=d.pop(key)
1 d = {}
1 if search in d: # O(1)
508 if n not in memo:
20 closers = d.keys() # cf.values()
105 d = {val:idx for idx, val in enumerate(inorder)}
383 from collections import Counter
383 d = Counter(magazine)
49 d = defaultdict(list)
49 l = [0]*26
49 d[tuple(l)].append(word)
6 d = {(0,0):s[0]} # key: (row,col), value: char
6 d[(row, col)] = c
1971 from collections import defaultdict
1971 graph = defaultdict(list)
1971 graph[u].append(v)
# 3.7+ Dictionary order is guaranteed to be insertion order by default
146 from collections import OrderedDict
146 self.cache.move_to_end(key)
146 self.cache.popitem(last=False) # cf. standard dictionaries does not accept any parameters
207 d = defaultdict(list) # d = {i:[] for i in range(numCourses)}
12 d1.update(d2)
12 for k, v in sorted(d1.items(), reverse=True):
208 d = {'T':{'r':{'i':{'e':{'<END>':''}}, 'y':{'<END>':''}}}} # Trie# add(), remove()
3 while s[r] in dup_chk:
3 dup_chk.remove(s[l])
14 s = set(t)
14 prefix.append(s.pop())
1971 visited = set([source])
1971 visited.add(adj_vertex)
128 starters = {num for num in nums if num-1 not in set_nums} # handle duplicates ahead49 dup_chk = set()
49 tmp = []
49 dup_chk.add(tuple(tmp)) # list is unhashable
2265 return (sum_vals, cnt_nodes)215 import heapq
215 heapify(l)
215 smallest_val = heappop(l)
347 h = [(-cnt, num) for num, cnt in counter.items()]
347 heapify(h) #O(n)
# Heap is an in-place algorithm, do not assign the return value. If h becomes None, it will cause an error to use len(h).
1046 h = heapify(h)
1584 min_heap = [(0,0)]
1584 dist, i = heappop(min_heap)
1584 heappush(min_heap, (manhattan_dist, j))200 def numIslands(self, grid: List[List[str]]) # matrix
1971 def validPath(self, n: int, edges: List[List[int]], source: int, destination: int) -> bool:
1971 edges = [[0,1],[1,2],[2,0]]
1971 graph = defaultdict(list)
for u, v in edges:
graph[u].append(v)
graph[v].append(u)
1971 curr = q.pop() # DFS: pop(), BFS: popleft()14 z = zip(*strs)
2418 l = sorted(list(zip(names, heights)), key=lambda item: item[1], reverse=True)704 l,r = 0, len(nums)-1
704 while l <= r:
215 for _ in range(k):
509 for i in range(1, n): # n=1: range(1,1) does not loop
1971 for u,v in edges:
1051 return sum(1 for i in range(n) if heights[i]!=expected[i])
172 from functools import reduce
172 factorial = reduce(lambda a,b: a*b, range(1, n+1), 1)
443 changed_cnt = sum(1 for i in range(8) if gene[i] != candidate[i])344 s[l], s[r] = s[r], s[l]
266 root.left, root.right = root.right, root.left
48 matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
189 nums[:k], nums[k:] = nums[-k:], nums[:-k]70 raise ValueError (f"1 <= n <= 45, n={n}")
146 if not isinstance(capacity, int) or capacity <= 0:
146 raise ValueError(f"capacity must be a positive integer. capacity={capacity}")
import traceback
except Exception as e:
traceback.print_exc()67 carry, r = divmod(n1 + n2 + carry,2)
49 l[ord(c)-ord('a')] += 1 # ord() is inverse of chr() which means ordinal, returns Unicode.
209 min_length = min(min_length, r-l+1)207 if not prerequisites: # if prerequisites is None or len(prerequisites) == 0:
# reverse of the logical state of an expression.
>>> not True
False
>>> not False
True
# check empty?
>>> not None
True
>>> not 0 # caution!!
True
>>> not 0.0 # caution!!
True
>>> not ""
True
>>> not ''
True
>>> not []
True
>>> not {}
True
>>> not set()
True
>>> from collections import deque
>>> not deque([])
True1268 from typing import List
1268 def build(self, words:List[str])-> None:380 from random import randint
380 idx = random.randint(0, len(self.dummy)-1)200 Stack overvlow limit in leetcode is 998
1051 For arrays of size 63 or less, Tim sort uses insertion sort.77 from functools import lru_cache
77 @lru_cache(None)RecursionError: maximum recursion depth exceeded in comparison
200 grid[row][col] == "0" # Fix: grid[row][col] = "0"
912 pivot = len(nums)//2 # Fix: pivot = nums[len(nums)//2]Invalid Syntax
NameError: Name mergeTwoLIsts is not defined
TypeError: Solution.mergeTwoLists() takes 2 positional arguments but 3 were given
23 def mergeTwoLIsts (list1, list2) # Fix: mergeTwoLIsts (self, list1, list2)TypeError: divmod expected 2 arguments, got 1
67 carry, r = divmod(n1 + n2 + carry) # Fix: carry, r = divmod(n1 + n2 + carry, 2)AttributeError: 'NoneType' object has no attribute 'left'
48 dfs(node.left, depth+1)