Bit Manipulation

Masks, shifts, and XOR identities.

Core syntax

• Test bit i — n & (1 << i).
• Lowest set bit — n & (-n); clear it — n & (n - 1).
• Count bits — n.bit_count() (3.10+) or bin(n).count('1').

missing = 0
for i, x in enumerate(nums):
    missing ^= i ^ x
missing ^= len(nums)          # XOR cancels pairs

Watch out

• x ^ x == 0 and x ^ 0 == x — the whole trick behind XOR puzzles.