Count number of 1's at specific position in a Matrix

Last Updated : 23 Jul, 2025

Given a binary matrix mat[][] of size m*n (0-based indexing), find the number of 1's such that 1 is located at a specific position where the same row and same column don't have any other 1's.

Examples:

Input: mat[][] = [[0, 0, 1], [0, 1, 0], [1, 0, 0]]
Output: 3
Explanation: All 1's in the matrix satisfy the given condition.

Input: mat[][] = [[1, 0, 1], [0, 1, 0], [1, 0, 0]]
Output: 1
Explanation: mat[1][1] satisfy the given condition.

Approach: The approach is based on matrix traversal.

Below are the steps for the above approach:

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).
Run a loop from i = 0 to i < m and check if row[i] == 1,
- Run a loop from j = 0 to j < n, and check if the element at the current position is 1 in mat[][] if mat[i][j] == 1
  - Check if cols[j] == 1, increment ans, else break.
Return ans.

Below is the code for the above approach:

C++

// C++ code for the above approach:
#include <bits/stdc++.h>
using namespace std;

int findNumberOfOnes(vector<vector<char> >& mat)
{
    const int m = mat.size();
    const int n = mat[0].size();
    int ans = 0;

    // Auxiliary row and col for tracking
    // how many B at that corresponding
    // row and col
    vector<int> rows(m);
    vector<int> cols(n);

    // 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 < m; ++i)
        for (int j = 0; j < n; ++j)
            if (mat[i][j] == 1) {
                ++rows[i];
                ++cols[j];
            }

    // Traversing through the mat[][]
    // matrix again and if
    // rows[i]==1 || cols[j]==1
    // then ans++, else continue.
    for (int i = 0; i < m; ++i)
        if (rows[i] == 1)
            for (int j = 0; j < n; ++j)

                // If the current position
                // is 1 in this row then
                // break and search
                // the next row
                if (mat[i][j] == 1) {
                    if (cols[j] == 1)
                        ++ans;
                    break;
                }
    return ans;
}

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

    // Function Call
    cout << findNumberOfOnes(mat);
    return 0;
}

Java

/*package whatever // do not write package name here */

// A program for finding the lonely color
import java.io.*;

class GFG {
    public static int findLonelyColor(char[][] painting)
    {
        int m = painting.length, n = painting[0].length;
        // Auxiliary row and col for tracking how many B at
        // that corresponding row and col
        int[] rows = new int[m];
        int[] cols = new int[n];

        // Traversing through the painting matrix. If
        // painting[i][j]==B then increment rows[i]++(for
        // row) and increment cols[j]++(for column).
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (painting[i][j] == 'B') {
                    rows[i]++;
                    cols[j]++;
                }
            }
        }

        int ans = 0;
        // Traversing through the painting matrix again and
        // if rows[i]==1 || cols[j]==1 then ans++, else
        // continue.
        for (int i = 0; i < m; ++i) {
            if (rows[i] == 1) {
                for (int j = 0; j < n; ++j) {
                    // If the current position is 'B' in this
                    // row then break and search the next
                    // row
                    if (painting[i][j] == 'B'
                        && cols[j] == 1) {
                        ans++;
                        break;
                    }
                }
            }
        }
        return ans;
    }

    public static void main(String[] args)
    {
        char[][] painting = { { 'W', 'W', 'B' },
                              { 'W', 'B', 'W' },
                              { 'B', 'W', 'W' } };
        int ans = findLonelyColor(painting);
        System.out.println(ans);
    }
}

Python3

# A program for finding the lonely color
from typing import List


def findLonelyColor(self, painting: List[List[str]]) -> int:
    # Auxiliary row and col for tracking how many B at that
    # corresponding row and col
    m, n = len(painting), len(painting[0])
    rows, cols = [0] * m, [0] * n
    # Traversing through the painting matrix.
    # If painting[i][j]== B then increment rows[i]++(for row)
    # and increment cols[j]++(for column).
    for i in range(m):
        for j in range(n):
            if painting[i][j] == 'B':
                rows[i] += 1
                cols[j] += 1

    ans = 0
    # Traversing through the painting matrix again and
    # if rows[i]== 1 || cols[j]== 1 then ans++, else continue
    for i in range(m):
        if rows[i] == 1:
            for j in range(n):
                # If the current position is 'B' in this row
                # then break and search the next row
                if painting[i][j] == 'B' and cols[j] == 1:
                    ans += 1
                    break
    return ans


if __name__ == "__main__":
    painting = [['W', 'W', 'B'],
                ['W', 'B', 'W'],
                ['B', 'W', 'W']]
    print(findLonelyColor(__name__, painting))

using System;

public class GFG {
    public static int FindLonelyColor(char[, ] painting)
    {
        int m = painting.GetLength(0);
        int n = painting.GetLength(1);
        // Auxiliary row and col for tracking how many B at
        // that corresponding row and col
        int[] rows = new int[m];
        int[] cols = new int[n];

        // Traversing through the painting matrix. If
        // painting[i,j] == 'B' then increment rows[i]++(for
        // row) and increment cols[j]++(for column).
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (painting[i, j] == 'B') {
                    rows[i]++;
                    cols[j]++;
                }
            }
        }

        int ans = 0;
        // Traversing through the painting matrix again and
        // if rows[i]==1 || cols[j]==1 then ans++, else
        // continue.
        for (int i = 0; i < m; ++i) {
            if (rows[i] == 1) {
                for (int j = 0; j < n; ++j) {
                    // If the current position is 'B' in this
                    // row then break and search the next
                    // row
                    if (painting[i, j] == 'B'
                        && cols[j] == 1) {
                        ans++;
                        break;
                    }
                }
            }
        }
        return ans;
    }

    public static void Main()
    {
        char[, ] painting = { { 'W', 'W', 'B' },
                              { 'W', 'B', 'W' },
                              { 'B', 'W', 'W' } };
        int ans = FindLonelyColor(painting);
        Console.WriteLine(ans);
    }
}
// This code is contributed by Prajwal Kandekar

JavaScript

// JavaScript code for the above approach:

function findNumberOfOnes(mat) {
  const m = mat.length;
  const n = mat[0].length;
  let ans = 0;

  // Auxiliary row and col for tracking
  // how many B at that corresponding
  // row and col
  const rows = new Array(m).fill(0);
  const cols = new Array(n).fill(0);

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

  // Traversing through the mat[][]
  // matrix again and if
  // rows[i]==1 || cols[j]==1
  // then ans++, else continue.
  for (let i = 0; i < m; ++i) {
    if (rows[i] === 1) {
      for (let j = 0; j < n; ++j) {
        // If the current position
        // is 1 in this row then
        // break and search
        // the next row
        if (mat[i][j] === 1) {
          if (cols[j] === 1) {
            ++ans;
          }
          break;
        }
      }
    }
  }
  return ans;
}

// Drivers code
const mat = [
  [0, 0, 1],
  [0, 1, 0],
  [1, 0, 0],
];

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

Output :

Time Complexity: O(m*n + m*n), Where m and n are the sizes of rows and columns
Auxiliary Space: O(m + n), used auxiliary array rows and cols of size m and n respectively

cosmic

Improve

Article Tags :

Count number of 1's at specific position in a Matrix

Output :

Explore

DSA Fundamentals

Data Structures

Algorithms

Advanced

Interview Preparation

Practice Problem

Thank You!

What kind of Experience do you want to share?