Given a Boolean Matrix, find k such that all elements in k'th row are 0 and k'th column are 1.
Last Updated :
08 Dec, 2022
Given a square boolean matrix mat[n][n], find k such that all elements in k'th row are 0 and all elements in k'th column are 1. The value of mat[k][k] can be anything (either 0 or 1). If no such k exists, return -1.
Examples:
Input: bool mat[n][n] = { {1, 0, 0, 0},
{1, 1, 1, 0},
{1, 1, 0, 0},
{1, 1, 1, 0},
};
Output: 0
All elements in 0'th row are 0 and all elements in
0'th column are 1. mat[0][0] is 1 (can be any value)
Input: bool mat[n][n] = {{0, 1, 1, 0, 1},
{0, 0, 0, 0, 0},
{1, 1, 1, 0, 0},
{1, 1, 1, 1, 0},
{1, 1, 1, 1, 1}};
Output: 1
All elements in 1'st row are 0 and all elements in
1'st column are 1. mat[1][1] is 0 (can be any value)
Input: bool mat[n][n] = {{0, 1, 1, 0, 1},
{0, 0, 0, 0, 0},
{1, 1, 1, 0, 0},
{1, 0, 1, 1, 0},
{1, 1, 1, 1, 1}};
Output: -1
There is no k such that k'th row elements are 0 and
k'th column elements are 1.
A Simple Solution is check all rows one by one. If we find a row 'i' such that all elements of this row are 0 except mat[i][i] which may be either 0 or 1, then we check all values in column 'i'. If all values are 1 in the column, then we return i. Time complexity of this solution is O(n2).
An Efficient Solution can solve this problem in O(n) time. The solution is based on below facts.
- There can be at most one k that can be qualified to be an answer (Why? Note that if k'th row has all 0's probably except mat[k][k], then no column can have all 1')s.
- If we traverse the given matrix from a corner (preferably from top right and bottom left), we can quickly discard complete row or complete column based on below rules.
- If mat[i][j] is 0 and i != j, then column j cannot be the solution.
- If mat[i][j] is 1 and i != j, then row i cannot be the solution.
Below is the complete algorithm based on above observations.
1) Start from top right corner, i.e., i = 0, j = n-1.
Initialize result as -1.
2) Do following until we find the result or reach outside the matrix.
......a) If mat[i][j] is 0, then check all elements on left of j in current row.
.........If all elements on left of j are also 0, then set result as i. Note
.........that i may not be result, but if there is a result, then it must be i
.........(Why? we reach mat[i][j] after discarding all rows above it and all
.........columns on right of it)
.........If all left side elements of i'th row are not 0, them this row cannot
.........be a solution, increment i.
......b) If mat[i][j] is 1, then check all elements below i in current column.
.........If all elements below i are 1, then set result as j. Note that j may
......... not be result, but if there is a result, then it must be j
.........If all elements of j'th column are not 1, them this column cannot be a
.........solution decrement j.
3) If result is -1, return it.
4) Else check validity of result by checking all row and column
elements of result
Below is the implementation based on the above idea.
C++
// C++ program to find i such that all entries in i'th row are 0
// and all entries in i't column are 1
#include <iostream>
using namespace std;
#define n 5
int find(bool arr[n][n])
{
// Start from top-most rightmost corner
// (We could start from other corners also)
int i=0, j=n-1;
// Initialize result
int res = -1;
// Find the index (This loop runs at most 2n times, we either
// increment row number or decrement column number)
while (i<n && j>=0)
{
// If current element is 0, then this row may be a solution
if (arr[i][j] == 0)
{
// Check for all elements in this row
while (j >= 0 && (arr[i][j] == 0 || i == j))
j--;
// If all values are 0, then store this row as result
if (j == -1)
{
res = i;
break;
}
// We reach here if we found a 1 in current row, so this
// row cannot be a solution, increment row number
else i++;
}
else // If current element is 1
{
// Check for all elements in this column
while (i<n && (arr[i][j] == 1 || i == j))
i++;
// If all elements are 1
if (i == n)
{
res = j;
break;
}
// We reach here if we found a 0 in current column, so this
// column cannot be a solution, increment column number
else j--;
}
}
// If we could not find result in above loop, then result doesn't exist
if (res == -1)
return res;
// Check if above computed res is valid
for (int i=0; i<n; i++)
if (res != i && arr[i][res] != 1)
return -1;
for (int j=0; j<n; j++)
if (res != j && arr[res][j] != 0)
return -1;
return res;
}
/* Driver program to test above functions */
int main()
{
bool mat[n][n] = {{0, 0, 1, 1, 0},
{0, 0, 0, 1, 0},
{1, 1, 1, 1, 0},
{0, 0, 0, 0, 0},
{1, 1, 1, 1, 1}};
cout << find(mat);
return 0;
}
// Java program to find i such that all entries in i'th row are 0
// and all entries in i't column are 1
import java.io.*;
public class GFG {
static int n = 5;
static int find(boolean arr[][]) {
// Start from top-most rightmost corner
// (We could start from other corners also)
int i = 0, j = n - 1;
// Initialize result
int res = -1;
// Find the index (This loop runs at most 2n times, we either
// increment row number or decrement column number)
while (i < n && j >= 0) {
// If current element is false, then this row may be a solution
if (arr[i][j] == false) {
// Check for all elements in this row
while (j >= 0 && (arr[i][j] == false || i == j)) {
j--;
}
// If all values are false, then store this row as result
if (j == -1) {
res = i;
break;
} // We reach here if we found a 1 in current row, so this
// row cannot be a solution, increment row number
else {
i++;
}
} else // If current element is 1
{
// Check for all elements in this column
while (i < n && (arr[i][j] == true || i == j)) {
i++;
}
// If all elements are 1
if (i == n) {
res = j;
break;
} // We reach here if we found a 0 in current column, so this
// column cannot be a solution, increment column number
else {
j--;
}
}
}
// If we could not find result in above loop, then result doesn't exist
if (res == -1) {
return res;
}
// Check if above computed res is valid
for (int k = 0; k < n; k++) {
if (res != k && arr[k][res] != true) {
return -1;
}
}
for (int l = 0; l < n; l++) {
if (res != l && arr[res][l] != false) {
return -1;
}
}
return res;
}
/* Driver program to test above functions */
public static void main(String[] args) {
boolean mat[][] = {{false, false, true, true, false},
{false, false, false, true, false},
{true, true, true, true, false},
{false, false, false, false, false},
{true, true, true, true, true}};
System.out.println(find(mat));
}
}
/* This Java code is contributed by PrinciRaj1992*/
''' Python program to find k such that all elements in k'th row
are 0 and k'th column are 1'''
def find(arr):
# store length of the array
n = len(arr)
# start from top right-most corner
i = 0
j = n - 1
# initialise result
res = -1
# find the index (This loop runs at most 2n times, we
# either increment row number or decrement column number)
while i < n and j >= 0:
# if the current element is 0, then this row may be a solution
if arr[i][j] == 0:
# check for all the elements in this row
while j >= 0 and (arr[i][j] == 0 or i == j):
j -= 1
# if all values are 0, update result as row number
if j == -1:
res = i
break
# if found a 1 in current row, the row can't be a
# solution, increment row number
else: i += 1
# if the current element is 1
else:
#check for all the elements in this column
while i < n and (arr[i][j] == 1 or i == j):
i +=1
# if all elements are 1, update result as col number
if i == n:
res = j
break
# if found a 0 in current column, the column can't be a
# solution, decrement column number
else: j -= 1
# if we couldn't find result in above loop, result doesn't exist
if res == -1:
return res
# check if the above computed res value is valid
for i in range(0, n):
if res != i and arr[i][res] != 1:
return -1
for j in range(0, j):
if res != j and arr[res][j] != 0:
return -1;
return res;
# test find(arr) function
arr = [ [0,0,1,1,0],
[0,0,0,1,0],
[1,1,1,1,0],
[0,0,0,0,0],
[1,1,1,1,1] ]
print (find(arr))
// C# program to find i such that all entries in i'th row are 0
// and all entries in i't column are 1
using System;
public class GFG{
static int n = 5;
static int find(bool [,]arr) {
// Start from top-most rightmost corner
// (We could start from other corners also)
int i = 0, j = n - 1;
// Initialize result
int res = -1;
// Find the index (This loop runs at most 2n times, we either
// increment row number or decrement column number)
while (i < n && j >= 0) {
// If current element is false, then this row may be a solution
if (arr[i,j] == false) {
// Check for all elements in this row
while (j >= 0 && (arr[i,j] == false || i == j)) {
j--;
}
// If all values are false, then store this row as result
if (j == -1) {
res = i;
break;
} // We reach here if we found a 1 in current row, so this
// row cannot be a solution, increment row number
else {
i++;
}
} else // If current element is 1
{
// Check for all elements in this column
while (i < n && (arr[i,j] == true || i == j)) {
i++;
}
// If all elements are 1
if (i == n) {
res = j;
break;
} // We reach here if we found a 0 in current column, so this
// column cannot be a solution, increment column number
else {
j--;
}
}
}
// If we could not find result in above loop, then result doesn't exist
if (res == -1) {
return res;
}
// Check if above computed res is valid
for (int k = 0; k < n; k++) {
if (res != k && arr[k,res] != true) {
return -1;
}
}
for (int l = 0; l < n; l++) {
if (res != l && arr[res,l] != false) {
return -1;
}
}
return res;
}
/* Driver program to test above functions */
public static void Main() {
bool [,]mat = {{false, false, true, true, false},
{false, false, false, true, false},
{true, true, true, true, false},
{false, false, false, false, false},
{true, true, true, true, true}};
Console.WriteLine(find(mat));
}
}
// This code is contributed by PrinciRaj1992
<?php
// PHP program to find i such that all
// entries in i'th row are 0 and all
// entries in i'th column are 1
function find(&$arr)
{
$n = 5;
// Start from top-most rightmost corner
// (We could start from other corners also)
$i = 0;
$j = $n - 1;
// Initialize result
$res = -1;
// Find the index (This loop runs at most
// 2n times, we either increment row number
// or decrement column number)
while ($i < $n && $j >= 0)
{
// If current element is 0, then this
// row may be a solution
if ($arr[$i][$j] == 0)
{
// Check for all elements in this row
while ($j >= 0 && ($arr[$i][$j] == 0 ||
$i == $j))
$j--;
// If all values are 0, then store
// this row as result
if ($j == -1)
{
$res = $i;
break;
}
// We reach here if we found a 1 in current
// row, so this row cannot be a solution,
// increment row number
else
$i++;
}
else // If current element is 1
{
// Check for all elements in this column
while ($i < $n && ($arr[$i][$j] == 1 ||
$i == $j))
$i++;
// If all elements are 1
if ($i == $n)
{
$res = $j;
break;
}
// We reach here if we found a 0 in current
// column, so this column cannot be a solution,
// increment column number
else
$j--;
}
}
// If we could not find result in above
// loop, then result doesn't exist
if ($res == -1)
return $res;
// Check if above computed res is valid
for ($i = 0; $i < $n; $i++)
if ($res != $i && $arr[$i][$res] != 1)
return -1;
for ($j = 0; $j < $n; $j++)
if ($res != $j && $arr[$res][$j] != 0)
return -1;
return $res;
}
// Driver Code
$mat = array(array(0, 0, 1, 1, 0),
array(0, 0, 0, 1, 0),
array(1, 1, 1, 1, 0),
array(0, 0, 0, 0, 0),
array(1, 1, 1, 1, 1));
echo (find($mat));
// This code is contributed by Shivi_Aggarwal
?>
<script>
// JavaScript program to find i such that
// all entries in i'th row are 0
// and all entries in i't column are 1
var n = 5;
function find(arr) {
// Start from top-most rightmost corner
// (We could start from other corners also)
var i = 0, j = n - 1;
// Initialize result
var res = -1;
// Find the index (This loop runs at most 2n times, we either
// increment row number or decrement column number)
while (i < n && j >= 0) {
// If current element is false,
// then this row may be a solution
if (arr[i][j] == false) {
// Check for all elements in this row
while (j >= 0 && (arr[i][j] == false || i == j)) {
j--;
}
// If all values are false,
// then store this row as result
if (j == -1) {
res = i;
break;
} // We reach here if we found a
// 1 in current row, so this
// row cannot be a solution, increment row number
else {
i++;
}
} else // If current element is 1
{
// Check for all elements in this column
while (i < n && (arr[i][j] == true || i == j)) {
i++;
}
// If all elements are 1
if (i == n) {
res = j;
break;
} // We reach here if we found a 0
// in current column, so this
// column cannot be a solution,
// increment column number
else {
j--;
}
}
}
// If we could not find result in above loop,
// then result doesn't exist
if (res == -1) {
return res;
}
// Check if above computed res is valid
for (var k = 0; k < n; k++) {
if (res != k && arr[k][res] != true) {
return -1;
}
}
for (var l = 0; l < n; l++) {
if (res != l && arr[res][l] != false) {
return -1;
}
}
return res;
}
/* Driver program to test above functions */
var mat = [[false, false, true, true, false],
[false, false, false, true, false],
[true, true, true, true, false],
[false, false, false, false, false],
[true, true, true, true, true]];
document.write(find(mat));
</script>
Time complexity of this solution is O(n). Note that we traverse at most 2n elements in the main while loop.
Auxiliary Space: O(1)
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