Python

Heap

Heaps are complete binary trees where the value of each node must be no greater than (or less than) the value of its child nodes.

• Python only supports Min Heaps
• import heapq
• heapq.heapify(arr)
• heapq.heappop(arr)
• heapq.heappush(arr, x)
• heapq.nsmallest(k, arr, key=func) returns a list with the k smallest elements in the iterable arr based on a comparator function func
  • Runtime: $O(N \log k)$
• heapq.nlargest(k, arr, key=func)returns a list with the k largest elements in the iterable arr based on a comparator function func
  • Runtime: $O(N \log k)$

List Offsetting

You can offset with Python's enumerate function with list splitting.

enumerate(nums[offset::])

Dictionary

• Python Dictionary is a hash map.
• Lookup time: $O(n)$ worst when there are many collisions.

Alphanumeric Testing

• c.isalnum()

Making 2d Arrays

• using visited = [[False] * len(image[0])] * len(image) will not work
  • the rows will share the same memory and change in one will reflect on the others
• use arr = [[0 for i in range(cols)] for j in range(rows)]