Skip to content

1504. Count Submatrices With All Ones

Description

Given an m x n binary matrix mat, return the number of submatrices that have all ones.

 

Example 1:

Input: mat = [[1,0,1],[1,1,0],[1,1,0]]
Output: 13
Explanation: 
There are 6 rectangles of side 1x1.
There are 2 rectangles of side 1x2.
There are 3 rectangles of side 2x1.
There is 1 rectangle of side 2x2. 
There is 1 rectangle of side 3x1.
Total number of rectangles = 6 + 2 + 3 + 1 + 1 = 13.

Example 2:

Input: mat = [[0,1,1,0],[0,1,1,1],[1,1,1,0]]
Output: 24
Explanation: 
There are 8 rectangles of side 1x1.
There are 5 rectangles of side 1x2.
There are 2 rectangles of side 1x3. 
There are 4 rectangles of side 2x1.
There are 2 rectangles of side 2x2. 
There are 2 rectangles of side 3x1. 
There is 1 rectangle of side 3x2. 
Total number of rectangles = 8 + 5 + 2 + 4 + 2 + 2 + 1 = 24.

 

Constraints:

  • 1 <= m, n <= 150
  • mat[i][j] is either 0 or 1.

Solutions

Solution 1: Enumeration + Prefix Sum

We can enumerate the bottom-right corner $(i, j)$ of the matrix, and then enumerate the first row $k$ upwards. The width of the matrix with $(i, j)$ as the bottom-right corner in each row is $\min_{k \leq i} \textit{g}[k][j]$, where $\textit{g}[k][j]$ represents the width of the matrix with $(k, j)$ as the bottom-right corner in the $k$-th row.

Therefore, we can preprocess a 2D array $g[i][j]$, where $g[i][j]$ represents the number of consecutive $1$s from the $j$-th column to the left in the $i$-th row.

The time complexity is $O(m^2 \times n)$, and the space complexity is $O(m \times n)$. Here, $m$ and $n$ are the number of rows and columns of the matrix, respectively.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
class Solution:
    def numSubmat(self, mat: List[List[int]]) -> int:
        m, n = len(mat), len(mat[0])
        g = [[0] * n for _ in range(m)]
        for i in range(m):
            for j in range(n):
                if mat[i][j]:
                    g[i][j] = 1 if j == 0 else 1 + g[i][j - 1]
        ans = 0
        for i in range(m):
            for j in range(n):
                col = inf
                for k in range(i, -1, -1):
                    col = min(col, g[k][j])
                    ans += col
        return ans

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
    public int numSubmat(int[][] mat) {
        int m = mat.length, n = mat[0].length;
        int[][] g = new int[m][n];
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (mat[i][j] == 1) {
                    g[i][j] = j == 0 ? 1 : 1 + g[i][j - 1];
                }
            }
        }
        int ans = 0;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                int col = 1 << 30;
                for (int k = i; k >= 0 && col > 0; --k) {
                    col = Math.min(col, g[k][j]);
                    ans += col;
                }
            }
        }
        return ans;
    }
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
class Solution {
public:
    int numSubmat(vector<vector<int>>& mat) {
        int m = mat.size(), n = mat[0].size();
        vector<vector<int>> g(m, vector<int>(n));
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (mat[i][j] == 1) {
                    g[i][j] = j == 0 ? 1 : 1 + g[i][j - 1];
                }
            }
        }
        int ans = 0;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                int col = 1 << 30;
                for (int k = i; k >= 0 && col > 0; --k) {
                    col = min(col, g[k][j]);
                    ans += col;
                }
            }
        }
        return ans;
    }
};

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
func numSubmat(mat [][]int) (ans int) {
    m, n := len(mat), len(mat[0])
    g := make([][]int, m)
    for i := range g {
        g[i] = make([]int, n)
        for j := range g[i] {
            if mat[i][j] == 1 {
                if j == 0 {
                    g[i][j] = 1
                } else {
                    g[i][j] = 1 + g[i][j-1]
                }
            }
        }
    }
    for i := range g {
        for j := range g[i] {
            col := 1 << 30
            for k := i; k >= 0 && col > 0; k-- {
                col = min(col, g[k][j])
                ans += col
            }
        }
    }
    return
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
function numSubmat(mat: number[][]): number {
    const m = mat.length;
    const n = mat[0].length;
    const g: number[][] = Array.from({ length: m }, () => Array(n).fill(0));

    for (let i = 0; i < m; i++) {
        for (let j = 0; j < n; j++) {
            if (mat[i][j]) {
                g[i][j] = j === 0 ? 1 : 1 + g[i][j - 1];
            }
        }
    }

    let ans = 0;
    for (let i = 0; i < m; i++) {
        for (let j = 0; j < n; j++) {
            let col = Infinity;
            for (let k = i; k >= 0; k--) {
                col = Math.min(col, g[k][j]);
                ans += col;
            }
        }
    }

    return ans;
}

Comments