README.md

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 (row1, col1) and lower right corner (row2, col2).

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 row1 ≤ row2 and col1 ≤ col2.

Solutions (Rust)

1. Solution

struct NumMatrix {
    mat: Vec<Vec<i32>>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl NumMatrix {
    fn new(matrix: Vec<Vec<i32>>) -> Self {
        let mut mat = matrix.clone();

        for r in 0..matrix.len() {
            let mut row_sum = 0;

            for c in 0..matrix[0].len() {
                if r > 0 {
                    mat[r][c] += mat[r - 1][c];
                }
                mat[r][c] += row_sum;
                row_sum += matrix[r][c];
            }
        }

        Self { mat }
    }

    fn sum_region(&self, row1: i32, col1: i32, row2: i32, col2: i32) -> i32 {
        let row1 = row1 as usize;
        let col1 = col1 as usize;
        let row2 = row2 as usize;
        let col2 = col2 as usize;

        self.mat[row2][col2]
            + match (row1 > 0, col1 > 0) {
                (false, false) => 0,
                (true, false) => -self.mat[row1 - 1][col2],
                (false, true) => -self.mat[row2][col1 - 1],
                _ => {
                    self.mat[row1 - 1][col1 - 1]
                        - self.mat[row1 - 1][col2]
                        - self.mat[row2][col1 - 1]
                }
            }
    }
}

/**
 * Your NumMatrix object will be instantiated and called as such:
 * let obj = NumMatrix::new(matrix);
 * let ret_1: i32 = obj.sum_region(row1, col1, row2, col2);
 */