algoclub 2/2/2025

find-pivot-index.py

class Solution:
    def pivotIndex(self, nums: List[int]) -> int:
        s = sum(nums)
        s2 = 0
        for i, x in enumerate(nums):
            if s2 == s - s2 - x: return i
            s2 += x
        return -1

range-sum-query-immutable.py

class NumArray:
    def __init__(self, nums):
        self.nums = [0] + nums
        for i in range(1, len(self.nums)):
            self.nums[i] += self.nums[i - 1]
        
    def sumRange(self, left: int, right: int) -> int:
        return self.nums[right+1] - self.nums[left]

subarray-sum-equals-k.py

class Solution:
    def subarraySum(self, nums, k):
        count = {0: 1}
        answer = 0
        s = 0
        for x in nums:
            s += x
            answer += count.get(s - k, 0)
            count[s] = count.get(s, 0) + 1
        return answer

subarray-sums-divisible-by-k.py

class Solution:
    def subarraysDivByK(self, nums: List[int], k: int) -> int:
        count = [1] + k * [0]
        answer = 0
        s = 0
        for x in nums:
            s += x
            answer += count[(s - k) % k]
            count[s % k] += 1
        return answer