CareerCup

static void AllCombinations(int Sum, int n)
{
    AllCombinations(Sum, n, "");
}
private static void AllCombinations(int sum, int n, string output)
{
    if (n == 1)
    {   
        Console.WriteLine(output+sum);
    }
    else
    {
        for(int i=sum; i>=0; i--)
        {
            string new_output = output + i + ",";
            AllCombinations(sum - i, n - 1, new_output);
        }
    }
}

Recursive C# code for this question.
Explanation on comments in-line.

static bool Search(int num, int[,]matrix,int m,int n) //m: number of rows, n: number of columns
        {
            if (num < matrix[0, 0] || num > matrix[m - 1, n - 1]) return false;
            return SearchUtil(num, matrix,0, m, 0, n);
        }
static bool SearchUtil(int num, int[,]matrix,int minI,int maxI, int minJ,int maxJ)
        {
            while (minI<=maxI)
            {
                int midI = (minI + maxI) / 2;
                int midJ = (minJ + maxJ) / 2;

                if (matrix[midI, midJ] == num)
                    return true;

                if (num > matrix[midI, midJ]) //If the number is greater than the middle element, it's either on the same row on middle's right side, or the next rows anywere.
                {
                    if (num <= matrix[midI, maxJ - 1]) //if it's smaller or equal to the largest element in this row; 
                    {
                        return SearchUtil(num, matrix, midI, midI, midJ, maxJ); //it's on the same row, to the right of the middle element.
                    }
                    else //if it's bigger than the middle element;
                    {
                        return SearchUtil(num, matrix, midI + 1, maxI, 0, maxJ); //it's for sure on one of the next rows.
                    }
                }

                if (num < matrix[midI, midJ]) //If the number is smaller than the middle element, it's either on the same row on middle's left side, or the previous rows anywere.
                {
                    if (num >= matrix[midI, 0]) //if it's greater or equal to the smallest element in this row; 
                    {
                        return SearchUtil(num, matrix, midI, midI, 0, midJ); //it's on the same row, to the left of the middle element.
                    }
                    else
                    {
                        return SearchUtil(num, matrix, 0, midI - 1, 0, maxJ); //it's for sure on one of the previous rows.
                    }
                } 
            }

            return false;
            
        }

Can you please explain more/provide examples?

Id did it with a min heap. I don't know how efficient it was but worked and they liked it!
I created a 'Word' class which store 'string word' and 'List<int> line_numbers'.

Please let me know your ideas and make sure to pay attention to the constraint 'Large Text File'.