304. Range Sum Query 2D - Immutable

Given a 2D matrix matrix , find the sum of the elements inside the rectangle defined by its upper left corner ( row 1, col 1) and lower right corner ( row 2, col 2).

The above rectangle (with the red border) is defined by (row1, col1) = (2, 1) and (row2, col2) = (4, 3) , which contains sum = 8.

Example:

Given matrix = [
  [3, 0, 1, 4, 2],
  [5, 6, 3, 2, 1],
  [1, 2, 0, 1, 5],
  [4, 1, 0, 1, 7],
  [1, 0, 3, 0, 5]
]

sumRegion(2, 1, 4, 3) -> 8
sumRegion(1, 1, 2, 2) -> 11
sumRegion(1, 2, 2, 4) -> 12

Note:

1. You may assume that the matrix does not change.
2. There are many calls to sumRegion function.
3. You may assume that row 1 ≤ row 2 and col 1 ≤ col 2.

# @lc code=start
using LeetCode

struct NumMatrix
    sums::Vector{Vector{Int}}
    function NumMatrix(matrix::Vector{Vector{Int}})
        m, n = length(matrix), length(matrix[1])
        sums = [fill(0, n + 1) for _ = 1:m]

        for i = 1:m, j = 1:n
            sums[i][j+1] = sums[i][j] + matrix[i][j]
        end

        new(sums)
    end
end

sum_region(matrix::NumMatrix, row1::Int, col1::Int, row2::Int, col2::Int) =
    sum(matrix.sums[i][col2+1] - matrix.sums[i][col1] for i = row1:row2)

# @lc code=end

sum_region (generic function with 1 method)

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