[LeetCode] Delete Columns to Make Sorted

Problem: You are given an array of n strings strs, all of the same length.

The strings can be arranged such that there is one on each line, making a grid. For example, strs = ["abc", "bce", "cae"] can be arranged as: 

abc
bce
cae

You want to delete the columns that are not sorted lexicographically. In the above example (0-indexed), columns 0 ('a', 'b', 'c') and 2 ('c', 'e', 'e') are sorted while column 1 ('b', 'c', 'a') is not, so you would delete column 1.

Return the number of columns that you will delete.

Example:

Input: strs = ["cba","daf","ghi"]
Output: 1
Explanation: The grid looks as follows:
  cba
  daf
  ghi
Columns 0 and 2 are sorted, but column 1 is not, so you only need to delete 1 column.

Input: strs = ["a","b"]
Output: 0
Explanation: The grid looks as follows:
  a
  b
Column 0 is the only column and is sorted, so you will not delete any columns.

Input: strs = ["zyx","wvu","tsr"]
Output: 3
Explanation: The grid looks as follows:
  zyx
  wvu
  tsr
All 3 columns are not sorted, so you will delete all 3.

Approach: This is a straightforward problem to solve (no tricks). We just need to check for every column if it is sorted or not. If not we can increase the count.

Implementation in C#:

    public int MinDeletionSize(string[] strs)

    {

        int numStrs = strs?.Length ?? 0;

        if (numStrs <= 1)

        {

            return 0;

        }

        int strLen = strs[0].Length;

        int numColsToDel = 0;

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

        {

            char prev = strs[0][j];

            for (int i = 1; i < numStrs; ++i)

            {

                if (prev > strs[i][j])

                {

                    ++numColsToDel;

                    break;

                }

                prev = strs[i][j];

            }

        }

        return numColsToDel;

    }

Complexity: O(m * n) where m is number of strings and n is length of string.