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Count number of 1's at specific position in a matrix - II

Last Updated : 23 Jul, 2025
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Given a binary matrix mat[][] of size m*n (0-based indexing), the task is to return the total number of 1's in the matrix that follows the below two rules. i.e:

  • Row R and column C both contain exactly N number of 1's.
  • All rows with 1's in column C should be the same as the 1's present in row R.

Examples:

Input: mat = [[0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 0, 1, 0, 1, 0]], N = 3
Output: 6
Explanation: All the underlined '1' are the 1's we need (all '1's at column 1 and 3). [[0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 0, 1, 0, 1, 0]]

  1. Rule 1: For each row R and column C, both should have exactly N number of "1's" (where N = 3 in this case).
  2. Rule 2: If the rows containing "1's" at column C are the same as row R, they contribute to the count.

Let's go through an example for row R = 0 and column C = 1:

  • Rule 1: Row R = 0 and column C = 1 both have exactly three "1's" in them.
  • Rule 2: The rows that have "1's" at column C = 1 are row 0, row 1, and row 2. These rows are exactly the same as row R = 0.
    • Total "1's" till now = 3

Next, we consider row R = 0 and column C = 2:

  • Rule 1: Row R = 0 and column C = 2 should both have exactly N number of "1's".
  • However, this violates Rule 1 since row R = 0 has three "1's" and column C = 2 has only one "1".
    • Total "1's" till now = 3 + 0

Finally, we consider row R = 0 and column C = 3:

  • Rule 1: Row R = 0 and column C = 3 both have exactly N = 3 "1's".
  • Rule 2: The rows that have "1's" at column C = 3 are row 0, row 1, and row 2. These rows are exactly the same as row R = 0.
    • Total "1's" till now = 3 + 0 + 3 = 6

We can apply this process for all rows and columns to determine the total count of "1's" following the given rules.

Input: mat = [[0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 1, 0, 1, 1, 0], [0, 0, 1, 0, 0, 0]], N = 3
Output: 9

Approach: Follow the steps to solve the problem:

The approach is based on matrix traversal.

Follow the steps to solve the problem:

  • Initialize auxiliary array rows[] and cols[] to track how many 1's are in the corresponding row or column.
  • Initialize a variable say, ans to store the number of 1's that satisfy the given condition.
  • Now traverse through the matrix mat[][] and check if mat[i][j]==1 then increment rows[i]++(for row) and increment cols[j]++(for column).
  • Create a boolean matrix of size row*row named 'same' for pre-calculating the row's equality
  • Run a loop from i = 0 to i < m,
    • Run a loop from j = 0 to j < n, and check if mat[i] == mat[j], if yes then mark true for the position same[i][j]==same[j][i] in the boolean matrix
  • Run a loop from i = 0 to i < m,
    • Run a loop from j = 0 to j < n,
      • check if the position of the 1 in this coordinate is the same as all rows with 1's in column C as well as the same in row R, if yes res++
  • return res

Below is the code for the above approach:

C++ 
// CPP program to find the Number of ones
// in the binary matrix while satisfying
// some conditions
#include <bits/stdc++.h>
using namespace std;

// If the position of the 1 in this
// coordinate is the same .as all rows
// with 1's in column C as well as the
// same in row R return true else
// return false,
bool check(const vector<vector<int> >& mat, int x, int y,
           int N, const vector<int>& rows,
           const vector<int>& cols,
           vector<vector<bool> >& same)
{
    if (mat[x][y] != 1)
        return false;
    if (rows[x] != N)
        return false;
    if (cols[y] != N)
        return false;

    for (int i = 0; i < mat.size(); i++)
        if (mat[i][y] == 1 && !same[i][x])
            return false;
    return true;
}

int findNumberOfOnes(vector<vector<int> >& mat, int N)
{

    // Auxiliary row and col for tracking
    // how many B at that corresponding
    // row and col
    int R = mat.size(), C = mat[0].size();
    vector<int> rows(R, 0), cols(C, 0);

    // Traversing through the mat[][]
    // matrix. If mat[i][j]==B then
    // increment rows[i]++(for row) and
    // increment cols[j]++(for column).
    for (int i = 0; i < R; i++)
        for (int j = 0; j < C; j++)
            if (mat[i][j] == 1)
                rows[i]++, cols[j]++;

    // Create a boolean matrix of size
    // row*row named 'same' for pre-
    // calculating the row's equality
    vector<vector<bool> > same(R, vector<bool>(R, false));
    for (int i = 0; i < R; i++)
        for (int j = i; j < R; j++)
            if (mat[i] == mat[j])
                same[i][j] = same[j][i] = true;

    int res = 0;

    // Traverse the mat[][] matrix again
    // for checking if the position of
    // the 1 in this coordinate is the
    // same as all rows with 1's in column
    // C as well as the same in row R,
    for (int i = 0; i < R; i++)
        for (int j = 0; j < C; j++)

            // call check function if
            // returned true res++
            if (check(mat, i, j, N, rows, cols, same))
                res++;
    return res;
}

// Drivers code
int main()
{
    vector<vector<int> > mat = { { 0, 1, 0, 1, 1, 0 },
                                 { 0, 1, 0, 1, 1, 0 },
                                 { 0, 1, 0, 1, 1, 0 },
                                 { 0, 0, 1, 0, 1, 0 } };
    int N = 3;

    // Function Call
    cout << findNumberOfOnes(mat, N);
    return 0;
}
Java 
// Java program to find the Number of ones
// in the binary matrix while satisfying
// some conditions
import java.util.*;

public class GFG {
    // If the position of the 1 in this
    // coordinate is the same as all rows
    // with 1's in column C as well as the
    // same in row R, return true, else
    // return false.
    static boolean check(List<List<Integer> > mat, int x,
                         int y, int N, List<Integer> rows,
                         List<Integer> cols,
                         boolean[][] same)
    {
        if (mat.get(x).get(y) != 1)
            return false;
        if (!rows.get(x).equals(N))
            return false;
        if (!cols.get(y).equals(N))
            return false;

        for (int i = 0; i < mat.size(); i++)
            if (mat.get(i).get(y) == 1 && !same[i][x])
                return false;
        return true;
    }

    static int findNumberOfOnes(List<List<Integer> > mat,
                                int N)
    {
        int R = mat.size();
        int C = mat.get(0).size();
        List<Integer> rows
            = new ArrayList<>(Collections.nCopies(R, 0));
        List<Integer> cols
            = new ArrayList<>(Collections.nCopies(C, 0));

        // Traversing through the mat[][] matrix.
        // If mat[i][j] == 1, increment rows[i]++
        // (for row) and increment cols[j]++
        // (for column).
        for (int i = 0; i < R; i++) {
            for (int j = 0; j < C; j++) {
                if (mat.get(i).get(j) == 1) {
                    rows.set(i, rows.get(i) + 1);
                    cols.set(j, cols.get(j) + 1);
                }
            }
        }

        // Create a boolean matrix of size row*row
        // named 'same' for pre-calculating the
        // row's equality
        boolean[][] same = new boolean[R][R];
        for (int i = 0; i < R; i++) {
            for (int j = i; j < R; j++) {
                if (mat.get(i).equals(mat.get(j))) {
                    same[i][j] = same[j][i] = true;
                }
            }
        }

        int res = 0;

        // Traverse the mat[][] matrix again for
        // checking if the position of the 1 in
        // this coordinate is the same as all rows
        // with 1's in column C as well as the same
        // in row R
        for (int i = 0; i < R; i++) {
            for (int j = 0; j < C; j++) {
                // Call check function, if returned true,
                // increment res
                if (check(mat, i, j, N, rows, cols, same)) {
                    res++;
                }
            }
        }
        return res;
    }

    // Driver code
    public static void main(String[] args)
    {
        List<List<Integer> > mat = Arrays.asList(
            Arrays.asList(0, 1, 0, 1, 1, 0),
            Arrays.asList(0, 1, 0, 1, 1, 0),
            Arrays.asList(0, 1, 0, 1, 1, 0),
            Arrays.asList(0, 0, 1, 0, 1, 0));
        int N = 3;

        // Function Call
        System.out.println(findNumberOfOnes(mat, N));
    }
}

// This code is contributed by Susobhan Akhuli
Python3 
#A program to find the Number of ones
# in the binary matrix while satisfying
#some conditions

# If the position of the 1 in this
# coordinate is the same .as all rows
# with 1's in column C as well as the
# same in row R return true else
# return false,

def check(mat, x, y, N, rows, cols, same):
    if mat[x][y] != 1:
        return False
    if rows[x] != N:
        return False
    if cols[y] != N:
        return False

    for i in range(len(mat)):
        if mat[i][y] == 1 and not same[i][x]:
            return False
    return True

def findNumberOfOnes(mat, N):
    R = len(mat)
    C = len(mat[0])
    rows = [0] * R
    cols = [0] * C

    # Traversing through the mat[][] matrix.
    # If mat[i][j] == 1, then increment rows[i] (for row)
    # and increment cols[j] (for column).
    for i in range(R):
        for j in range(C):
            if mat[i][j] == 1:
                rows[i] += 1
                cols[j] += 1

    # Create a boolean matrix of size R x R named 'same'
    # for pre-calculating the row's equality
    same = [[False] * R for _ in range(R)]
    for i in range(R):
        for j in range(i, R):
            if mat[i] == mat[j]:
                same[i][j] = same[j][i] = True

    res = 0

    # Traverse the mat[][] matrix again for checking
    # if the position of the 1 in this coordinate is
    # the same as all rows with 1's in column C as well
    # as the same in row R.
    for i in range(R):
        for j in range(C):
            # Call the check function and if it returns True, increment res
            if check(mat, i, j, N, rows, cols, same):
                res += 1
    return res

mat = [
    [0, 1, 0, 1, 1, 0],
    [0, 1, 0, 1, 1, 0],
    [0, 1, 0, 1, 1, 0],
    [0, 0, 1, 0, 1, 0]
]
N = 3

# Function call
print(findNumberOfOnes(mat, N))
C# 
// C# program to find the Number of ones
// in the binary matrix while satisfying
// some conditions
using System;
using System.Collections.Generic;
using System.Linq;

public class GFG {
    // If the position of the 1 in this
    // coordinate is the same as all rows
    // with 1's in column C as well as the
    // same in row R, return true, else
    // return false.
    static bool Check(List<List<int> > mat, int x, int y,
                      int N, List<int> rows, List<int> cols,
                      bool[, ] same)
    {
        if (mat[x][y] != 1)
            return false;
        if (rows[x] != N)
            return false;
        if (cols[y] != N)
            return false;

        for (int i = 0; i < mat.Count; i++)
            if (mat[i][y] == 1 && !same[i, x])
                return false;
        return true;
    }

    static int FindNumberOfOnes(List<List<int> > mat, int N)
    {
        int R = mat.Count;
        int C = mat[0].Count;
        List<int> rows
            = new List<int>(Enumerable.Repeat(0, R));
        List<int> cols
            = new List<int>(Enumerable.Repeat(0, C));

        // Traversing through the mat[,] matrix.
        // If mat[i,j] == 1, increment rows[i]++
        // (for row) and increment cols[j]++
        // (for column).
        for (int i = 0; i < R; i++) {
            for (int j = 0; j < C; j++) {
                if (mat[i][j] == 1) {
                    rows[i]++;
                    cols[j]++;
                }
            }
        }

        // Create a boolean matrix of size row*row
        // named 'same' for pre-calculating the
        // row's equality
        bool[, ] same = new bool[R, R];
        for (int i = 0; i < R; i++) {
            for (int j = i; j < R; j++) {
                if (mat[i].SequenceEqual(mat[j])) {
                    same[i, j] = same[j, i] = true;
                }
            }
        }

        int res = 0;

        // Traverse the mat[,] matrix again for
        // checking if the position of the 1 in
        // this coordinate is the same as all rows
        // with 1's in column C as well as the same
        // in row R
        for (int i = 0; i < R; i++) {
            for (int j = 0; j < C; j++) {
                // Call Check function, if returned true,
                // increment res
                if (Check(mat, i, j, N, rows, cols, same)) {
                    res++;
                }
            }
        }
        return res;
    }

    // Driver code
    public static void Main(string[] args)
    {
        List<List<int> > mat = new List<List<int> >{
            new List<int>{ 0, 1, 0, 1, 1, 0 },
            new List<int>{ 0, 1, 0, 1, 1, 0 },
            new List<int>{ 0, 1, 0, 1, 1, 0 },
            new List<int>{ 0, 0, 1, 0, 1, 0 }
        };
        int N = 3;

        // Function Call
        Console.WriteLine(FindNumberOfOnes(mat, N));
    }
}

// This code is contributed by Susobhan Akhuli
JavaScript 
// JavaScript program to find the Number of ones
// in the binary matrix while satisfying
// some conditions

// Function to check if the position of 1 in this
// coordinate is the same as all rows with 1's in column C
// as well as the same in row R, return true, else return false.
function check(mat, x, y, N, rows, cols, same) {
    if (mat[x][y] !== 1) return false;
    if (rows[x] !== N) return false;
    if (cols[y] !== N) return false;

    for (let i = 0; i < mat.length; i++) {
        if (mat[i][y] === 1 && !same[i][x]) {
            return false;
        }
    }
    return true;
}

// Function to find the number of ones in a binary matrix
// while satisfying certain conditions
function findNumberOfOnes(mat, N) {
    const R = mat.length;
    const C = mat[0].length;

    // Arrays to track the number of 1's in rows and columns
    const rows = new Array(R).fill(0);
    const cols = new Array(C).fill(0);

    // Traversing through the matrix mat[][].
    // If mat[i][j] == 1, increment rows[i] (for row) and cols[j] (for column).
    for (let i = 0; i < R; i++) {
        for (let j = 0; j < C; j++) {
            if (mat[i][j] === 1) {
                rows[i]++;
                cols[j]++;
            }
        }
    }

    // Create a boolean matrix named 'same' of size row * row
    // for pre-calculating the row's equality
    const same = new Array(R).fill(false).map(() => new Array(R).fill(false));
    for (let i = 0; i < R; i++) {
        for (let j = i; j < R; j++) {
            if (JSON.stringify(mat[i]) === JSON.stringify(mat[j])) {
                same[i][j] = same[j][i] = true;
            }
        }
    }

    let res = 0;

    // Traverse the matrix mat[][] again
    // to check if the position of 1 in this coordinate
    // is the same as all rows with 1's in column C
    // as well as the same in row R
    for (let i = 0; i < R; i++) {
        for (let j = 0; j < C; j++) {
            // Call the check function, if it returns true, increment res
            if (check(mat, i, j, N, rows, cols, same)) {
                res++;
            }
        }
    }

    return res;
}

// Driver code
const mat = [
    [0, 1, 0, 1, 1, 0],
    [0, 1, 0, 1, 1, 0],
    [0, 1, 0, 1, 1, 0],
    [0, 0, 1, 0, 1, 0]
];
const N = 3;

// Function Call
console.log(findNumberOfOnes(mat, N));

Output
6

Time Complexity: O(R*C*(R2)), where R is the number of rows in the binary matrix and C is the number of columns in the binary matrix.
Auxiliary Space: O(R+C+R2)


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