class RangeSum:
    def __init__(self, nums):
        self.prefix = [0]
        for x in nums:
            self.prefix.append(self.prefix[-1] + x)

    def query(self, i, j):
        return self.prefix[j + 1] - self.prefix[i]


def subarray_sum_equals_k(nums, k):
    counts = {0: 1}
    running = 0
    total = 0
    for x in nums:
        running += x
        total += counts.get(running - k, 0)
        counts[running] = counts.get(running, 0) + 1
    return total


class DiffArray:
    def __init__(self, n):
        """Start a length-n logical array of zeros."""
        # TODO: keep an internal diff list of size n + 1
        pass

    def add(self, i, j, delta):
        """Add delta to every index in [i, j]."""
        # TODO: diff[i] += delta and diff[j + 1] -= delta
        pass

    def result(self):
        """Materialize the final array via a prefix sum of diff."""
        # TODO: running prefix sum over the first n diff entries
        pass