Question

3070. Count Submatrices with Top-Left Element and Sum Less Than k

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Description

You are given a 0-indexed integer matrix grid and an integer k.

Return _the number of submatrices that contain the top-left element of the_ grid, _and have a sum less than or equal to _k.

 

Example 1:

Input: grid = [[7,6,3],[6,6,1]], k = 18
Output: 4
Explanation: There are only 4 submatrices, shown in the image above, that contain the top-left element of grid, and have a sum less than or equal to 18.

Example 2:

Input: grid = [[7,2,9],[1,5,0],[2,6,6]], k = 20
Output: 6
Explanation: There are only 6 submatrices, shown in the image above, that contain the top-left element of grid, and have a sum less than or equal to 20.

 

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= n, m <= 1000
• 0 <= grid[i][j] <= 1000
• 1 <= k <= 109

Solutions

Solution 1: Two-Dimensional Prefix Sum

The problem is actually asking for the number of prefix submatrices in a two-dimensional matrix whose sum is less than or equal to $k$.

The calculation formula for the two-dimensional prefix sum is:

$$ s[i][j] = s[i-1][j] + s[i][j-1] - s[i-1][j-1] + x $$

The time complexity is $O(m \times n)$, and the space complexity is $O(m \times n)$, where $m$ and $n$ are the number of rows and columns of the matrix, respectively.

class Solution:
  def countSubmatrices(self, grid: List[List[int]], k: int) -> int:
    s = [[0] * (len(grid[0]) + 1) for _ in range(len(grid) + 1)]
    ans = 0
    for i, row in enumerate(grid, 1):
      for j, x in enumerate(row, 1):
        s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + x
        ans += s[i][j] <= k
    return ans

class Solution {
  public int countSubmatrices(int[][] grid, int k) {
    int m = grid.length, n = grid[0].length;
    int[][] s = new int[m + 1][n + 1];
    int ans = 0;
    for (int i = 1; i <= m; ++i) {
      for (int j = 1; j <= n; ++j) {
        s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + grid[i - 1][j - 1];
        if (s[i][j] <= k) {
          ++ans;
        }
      }
    }
    return ans;
  }
}

class Solution {
public:
  int countSubmatrices(vector<vector<int>>& grid, int k) {
    int m = grid.size(), n = grid[0].size();
    int s[m + 1][n + 1];
    memset(s, 0, sizeof(s));
    int ans = 0;
    for (int i = 1; i <= m; ++i) {
      for (int j = 1; j <= n; ++j) {
        s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + grid[i - 1][j - 1];
        if (s[i][j] <= k) {
          ++ans;
        }
      }
    }
    return ans;
  }
};

func countSubmatrices(grid [][]int, k int) (ans int) {
  s := make([][]int, len(grid)+1)
  for i := range s {
    s[i] = make([]int, len(grid[0])+1)
  }
  for i, row := range grid {
    for j, x := range row {
      s[i+1][j+1] = s[i+1][j] + s[i][j+1] - s[i][j] + x
      if s[i+1][j+1] <= k {
        ans++
      }
    }
  }
  return
}

function countSubmatrices(grid: number[][], k: number): number {
  const m = grid.length;
  const n = grid[0].length;
  const s: number[][] = Array.from({ length: m + 1 }, () => Array(n + 1).fill(0));
  let ans: number = 0;
  for (let i = 1; i <= m; ++i) {
    for (let j = 1; j <= n; ++j) {
      s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + grid[i - 1][j - 1];
      if (s[i][j] <= k) {
        ++ans;
      }
    }
  }
  return ans;
}