Computes the sum of elements within a specified range `[left, right]`. This operation calculates the range sum by performing two prefix sum queries. # Arguments `left` - The zero-based starting index of the range. `right` - The zero-based ending index of the range. # Returns A `Result` containing the range sum (`Ok(sum)`) or an error (`FenwickTreeError::InvalidRange`) if the left index is gre
(&self, left: usize, right: usize)
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