🌱 DSA (Beginner) · Core Patterns · cozy lesson
Prefix Sum
10 min · 1 min read · no scary math, promise
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pref = [0]
for x in a: pref.append(pref[-1]+x)
# sum l..r = pref[r+1]-pref[l]
def subarray_k(a, k):
from collections import Counter
c, cur, ans = Counter({0:1}), 0, 0
for x in a:
cur += x
ans += c[cur-k]
c[cur] += 1
return ans
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1. Range sum l..r?
2. Subarray sum k counts via?
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