Core Skill

• Precompute cumulative values
• repeated range queries
• pre-computation saves time

Problems

Product of Array Except Self

Subarray Sum Equals K

Pattern

prefix[i] = prefix[i-1] + nums[i]

prefix = [0] * (n+1)
for i in range(n):
    prefix[i+1] = prefix[i] + nums[i]

Insight

• Avoid recomputation → O(n²) → O(n)