944. Delete Columns to Make Sorted

Description

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 follows:

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.

 

Solutions

Solution: Two Pointers

• Time complexity: O(mn)
• Space complexity: O(1)

 

JavaScript

/**
 * @param {string[]} strs
 * @return {number}
 */
const minDeletionSize = function (strs) {
  const m = strs.length;
  const n = strs[0].length;
  let result = 0;

  const isStoredCol = col => {
    for (let row = 1; row < m; row++) {
      if (strs[row][col] < strs[row - 1][col]) {
        return false;
      }
    }

    return true;
  };

  for (let col = 0; col < n; col++) {
    if (!isStoredCol(col)) {
      result += 1;
    }
  }

  return result;
};