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1504. Count Submatrices With All Ones 👍

Approach 1: Naive

  • Time: $O(m^2n)$
  • Space: $O(1)$
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class Solution {
 public:
  int numSubmat(vector<vector<int>>& mat) {
    const int m = mat.size();
    const int n = mat[0].size();
    int ans = 0;

    for (int baseRow = 0; baseRow < m; ++baseRow) {
      vector<int> row(n, 1);
      for (int i = baseRow; i < m; ++i) {
        for (int j = 0; j < n; ++j)
          row[j] &= mat[i][j];
        ans += count(row);
      }
    }

    return ans;
  }

 private:
  int count(vector<int>& row) {
    int ans = 0;
    int length = 0;

    for (const int a : row) {
      length = a == 0 ? 0 : length + 1;
      ans += length;
    }

    return ans;
  }
};

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class Solution {
  public int numSubmat(int[][] mat) {
    final int m = mat.length;
    final int n = mat[0].length;
    int ans = 0;

    for (int baseRow = 0; baseRow < m; ++baseRow) {
      int[] row = new int[n];
      Arrays.fill(row, 1);
      for (int i = baseRow; i < m; ++i) {
        for (int j = 0; j < n; ++j)
          row[j] &= mat[i][j];
        ans += count(row);
      }
    }

    return ans;
  }

  private int count(int[] row) {
    int ans = 0;
    int length = 0;

    for (final int a : row) {
      length = a == 0 ? 0 : length + 1;
      ans += length;
    }

    return ans;
  }
}

Approach 2: Stack

  • Time: $O(mn)$
  • Space: $O(n)$
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class Solution {
 public:
  int numSubmat(vector<vector<int>>& mat) {
    const int m = mat.size();
    const int n = mat[0].size();
    int ans = 0;
    vector<int> hist(n);

    for (const vector<int>& row : mat) {
      for (int i = 0; i < row.size(); ++i)
        hist[i] = row[i] == 0 ? 0 : hist[i] + 1;
      ans += count(hist);
    }

    return ans;
  }

 private:
  int count(const vector<int>& A) {
    // submatrices[i] := the number of submatrices, where the i-th column is the
    // right border
    vector<int> submatrices(A.size());
    stack<int> stack;

    for (int i = 0; i < A.size(); ++i) {
      while (!stack.empty() && A[stack.top()] >= A[i])
        stack.pop();
      if (!stack.empty()) {
        const int prevIndex = stack.top();
        submatrices[i] = submatrices[prevIndex] + A[i] * (i - prevIndex);
      } else {
        submatrices[i] = A[i] * (i + 1);
      }
      stack.push(i);
    }

    return accumulate(submatrices.begin(), submatrices.end(), 0);
  }
};

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class Solution {
  public int numSubmat(int[][] mat) {
    final int m = mat.length;
    final int n = mat[0].length;
    int ans = 0;
    int[] hist = new int[n];

    for (int[] row : mat) {
      for (int i = 0; i < row.length; ++i)
        hist[i] = row[i] == 0 ? 0 : hist[i] + 1;
      ans += count(hist);
    }

    return ans;
  }

  private int count(int[] A) {
    // submatrices[i] := the number of submatrices, where the i-th column is the right border
    int[] submatrices = new int[A.length];
    Deque<Integer> stack = new ArrayDeque<>();

    for (int i = 0; i < A.length; ++i) {
      while (!stack.isEmpty() && A[stack.peek()] >= A[i])
        stack.pop();
      if (!stack.isEmpty()) {
        final int prevIndex = stack.peek();
        submatrices[i] = submatrices[prevIndex] + A[i] * (i - prevIndex);
      } else {
        submatrices[i] = A[i] * (i + 1);
      }
      stack.push(i);
    }

    return Arrays.stream(submatrices).sum();
  }
}