Before moving on, implement a BIT from scratch without looking at notes:
Implement update(i, delta) and query(i)
Build a BIT from an array
Test with range sum queries
Verify against brute force
Once you can write it in under 2 minutes, you've internalized the pattern.
# Template to fill in:
class BIT:
def __init__(self, n):
# Your code
def update(self, i, delta):
# Your code
def query(self, i):
# Your code
def rangeQuery(self, l, r):
# Your code
The best way to learn data structures is to implement them from memory.