## Microsoft Interview Question

Software Engineer / Developers- 0of 0 votes
Given an n X n array with rows sorted and cols sorted, find the number of negative elements in most efficient way

sorry it's not O(n)complexity. In the worst case you check all the n^2 elements of the matrix and so it's o(n^2). Instead we can do binary search on every row to make it O(nlog n). I wonder if we can make it run faster

Raj, why do you think worst case will be O(n^2)? The question simply asks for finding the number of negative elements.

The solution suggested by random does not work if we start from top right edge of the matrix, but it works fine if we start from bottom right and move upwards towards [0,0] element. Here is updated code

```
public int CountNumberofNegativeNumbersInASortedMatrix()
{
int N = 5;
int [,] arr =
{
{ -3, -2, 3, 4, 5},
{ -4, -3, 2, 3, 4},
{ -5, -4, -1, 1, 2},
{ -6, -5, -2, 0, 1},
{ -7, -6, -3, -2, -1},
};
if ( arr[0,0] >= 0 )
return 0;
if ( arr[N -1 ,N -1] < 0 )
return N -1 * N - 1;
int row = N - 1;
int col = N - 1;
int count = 0; //count is the number of negatives in the matrix
while( row >= 0 && col >= 0 )
{
if(arr[row,col] < 0)
{
count += col + 1 ; //everything on the left of a(row,col)including itself are -ve
row--;
}
else
{
col--;
}
}
return count;
}
```

In the worst-case, if there are no negative elements in the matrix, you need to traverse all rows and all columns and thus the complexity becomes O(n^2)

rough idea:

M row, N column

1.start from right-bottom corner, go up and check the 1st element <0, if not found, proceed to next column

2.when the 1st elem that < 0 is found, record it - say (i,j), and go downward to (i-1,j) and then go left and check the 1st elem <0;

repeat 1,2 and we are basically tracking the right-bottom position of the area with all -ve numbers.

worst case O(MN),

improvement can be done by using binary search when checking the 1st -ve number at each row/column

I would start with this. I wonder about complexity - is it really O(log N)?

```
static class Program
{
// Test data.
static int[,] array = { { -2, -1, 3 }, { -1, 0, 4 }, { 1, 3, 5 } };
// Counts the number of negative elements in a given (sub)rectangle.
static int CountNegatives(int row, int col, int rowCount, int colCount)
{
// Make sure the area is not empty.
if (rowCount == 0 || colCount == 0)
return 0;
// Check the bottom-right (largest) element.
if (array[row + rowCount - 1, col + colCount - 1] < 0)
return rowCount * colCount; // all numbers are negative
// Check the top-left (smallest) element.
if (array[row, col] >= 0)
return 0; // all numbers are positive
// Divide the area into 4 sub-rectangles and process them recursively.
int dRow = rowCount / 2;
int dCol = colCount / 2;
return
CountNegatives(row, col, dRow, dCol) +
CountNegatives(row + dRow, col, dRow, dCol) +
CountNegatives(row, col + dCol, dRow, dCol) +
CountNegatives(row + dRow, col + dCol, dRow, dCol);
}
static void Main(string[] args)
{
System.Console.WriteLine(CountNegatives(0, 0, array.GetLength(0), array.GetLength(1)));
}
}
```

it is no way O(logN).

Suppose N= 2^k.

You get c * (2 + 2*2 + 2* 2^2 + ...+ 2^K)= c*2^(K+1)=2c*N

where c is some overhead number.

^ there's an obvious bug there (the rightmost column and the bottom row will be sometimes lost because of the integer division by 2), but I guess the algorithm should work once that bug is fixed.

```
// Divide the area into 4 sub-rectangles and process them recursively.
int dRow = rowCount / 2;
int dCol = colCount / 2;
int rRow = rowCount % 2;
int rCol = colCount % 2;
return
CountNegatives(row, col, dRow, dCol) +
CountNegatives(row + dRow, col, dRow + rRow, dCol) +
CountNegatives(row, col + dCol, dRow, dCol + rCol) +
CountNegatives(row + dRow, col + dCol, dRow + rRow, dCol + rCol);
```

Given the array, a[n][n],the negative num is Num(Num=0 initally) from the right-top corner, a[0][n-1], check whether it >0 or not, if >=0, move to the lefter one, a[0][n-2], if it < 0, found the first element which >=0 in the same column, suppose it is a[x][n-1], Num+=x*n, Move to the a[x][n-2], do the similar things untill the current element move outside the array. The time complexisty is O(n)

#include<iostream>

using namespace std;

int findNeg(int Arr[][3])

{

int count = 0,COL=3;

for(int i=0;i<3;i++)

{

if(Arr[i][0]>0)

break;

for(int j=0;j<COL;j++)

{

if(Arr[i][j]>0)

{

COL = j;

break;

}

count++;

}

}

return count;

}

int main()

{

int arr[3][3] = {{-3,-2,-1},{-2,1,3},{1,2,4}};

cout <<"Number of Negative numbers = " <<findNeg(arr);

return 0;

}

Guys,

Well, it is definitely O(N) solution. One outstanding property is a diagonal property. So to say, Let cell X = (k,k) be a diagonal cell. No other cell (i,j); i < k, and j < k has a value greater than X. Then the solution is move diagonally from (0,0) bottom-right. Find the last diagonal that still negative, then we are guarantee to have at least (k-1)^2 negative cells. Then count the negative number in column k and row k. The time complexity is sqrt(2N^2) + 2N = O(N).

Approach: Assuming indexes in the range 1...N,start at (1,N) ,i.e. right hand top and check if the element is negative.If yes,obviously all the elements on its left including itself would be -ve.So set the count_of_negatives to N and increment the row_index.If a(1,N) is >=0,decrement the col_index.

- random December 14, 2009So at any general grid point , if a(row,col)< 0, increment row by 1 and count_of_negatives by col,because all the elements to the left of a(row,col) including itself in the same row would be < 0. On the other hand if a(row,col) >=0, just decrement col. Execute the above in a loop till you transcend the boundaries of the matrix.

Time Complexity is 2n which is O(n),where n is number of rows(cols) in the square matrix

The pseudocode is

numNegatives(a[1..N][1...N]){

if a(1,1) >= 0 return 0;

if a(N,N) < 0 return N*N;

row = 1

col = N

count = 0; //count is the number of negatives in the matrix

while( row <=N && col >= 1 ){

if(a(row,col) < 0){

count += col; //everything on the left of a(row,col)including itself are -ve

row++;

}else{

col--;

}

}

return count;

}