Small Snippets

Four-direction neighbors on a grid

rows = len(grid)
cols = len(grid[0])

dir = [(1, 0), (-1, 0), (0, 1), (0, -1)]

for dr, dc in dir:
    nr = row + dr
    nc = col + dc

    if 0 <= nr < rows and 0 <= nc < cols:
        # Valid neighbor cell
        pass

Prefix sum array

prefix = [0]

for value in nums:
    prefix.append(prefix[-1] + value)

# Sum nums[left:right + 1] (inclusive)
range_sum = prefix[right + 1] - prefix[left]

Graph from edge list

edges = [[0, 1], [1, 2]]
n = 3

adjacency = [[] for _ in range(n)]

for source, target in edges:
    adjacency[source].append(target)
    adjacency[target].append(source)  # omit for directed graph

Binary tree preorder (iterative)

values = []
stack = [root]

while stack:
    node = stack.pop()

    if not node:
        continue

    values.append(node.val)
    stack.append(node.right)
    stack.append(node.left)

Min-heap / top-K

import heapq

heap = []

for value in nums:
    heapq.heappush(heap, value)
    if len(heap) > k:
        heapq.heappop(heap)

# heap holds the k smallest elements; heap[0] is the largest among them