LeetCode 944. Delete Columns to Make Sorted

Description

https://leetcode.com/problems/delete-columns-to-make-sorted/

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 1:

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.

Example 2:

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.

Example 3:

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.

Constraints:

  • n == strs.length
  • 1 <= n <= 100
  • 1 <= strs[i].length <= 1000
  • strs[i] consists of lowercase English letters.

Explanation

Check column by column if sorted.

Python Solution

class Solution:
    def minDeletionSize(self, strs: List[str]) -> int:
        columns = defaultdict(list)
        
        for word in strs:
            for i in range(len(word)):
                ch = word[i]
                columns[i].append(ch)
                
        count = 0
        for key, value in columns.items():
            if value != sorted(value):
                count += 1
                
        return count
   
  • Time Complexity: O(N))
  • Space Complexity: O(N)

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