Problem Description

The problem provides an array of n strings, strs, with each string being of the same length. The goal is to determine the fewest number of index deletions required so that, after the deletions, the remaining letters within each string form a non-decreasing sequence in lexicographic (alphabetical) order.

To visualize, imagine each string as a row in a grid, with the columns aligned vertically. Deleting an index corresponds to removing a column from this grid. The challenge is to delete the fewest columns necessary so that the letters in each row of the grid strictly increase or stay the same as you move left to right.

The result of the process is a set of indices that, if deleted, would satisfy the condition for every string in the array to be lexicographically ordered from the first to the last character. The task is to return the minimum size of such a set of deletion indices.

Intuition

The approach to this problem involves dynamic programming. Dynamic programming is a strategy for solving problems by breaking them down into simpler subproblems and storing the solutions to these subproblems to avoid redundant work.

Here's the intuition behind the solution:

1. The problem resembles the classic problem of finding the longest increasing subsequence (LIS), but inverted. Instead of finding the longest subsequence, we're trying to find the minimum number of deletions, which correlates to finding the maximum length of an "ordered" subsequence that doesn't require deletion.

2. We initialize a dynamic programming array dp where dp[i] represents the length of the longest ordered subsequence ending with the ith character (inclusive).

3. We iterate over every pair i and j, where i > j. If the jth character is less than or equal to the ith character for all strings, it means we can extend the ordered subsequence ending at j to include i. We update dp[i] if this subsequence is longer than the current one.

4. The maximum value in the dp array represents the length of the longest possible ordered subsequence that we can obtain without any deletions. Therefore, we subtract this value from the total number of columns n to get the minimum number of deletions required.

This solution essentially finds the longest subsequence of columns that are already in non-decreasing order for all rows and removes the rest, minimizing the number of deletions required while achieving the goal.

Learn more about Dynamic Programming patterns.

Solution Approach

The implementation of the solution utilizes dynamic programming, which is evident from the use of the dp array where each dp[i] keeps track of the length of the longest non-decreasing subsequence of characters ending with the ith column. Let's walk through the approach step by step:

1. Define n to be the length of the strings, which is also the number of columns if each string is considered a row in a grid.

2. Initialize the dp array of size n with all elements set to 1. Each dp[i] represents the length of the longest non-decreasing subsequence of columns when considering columns 0 to i. Initially, we set them all to 1 since each column by itself can be a subsequence.

3. Use two nested loops with indices i and j, where i ranges from 1 to n-1 and j ranges from 0 to i-1. These loops are used to find the length of the longest non-decreasing subsequence ending at each column i.

4. For each pair of i and j, check if the jth column is less than or equal to the ith column for all strings (all(s[j] <= s[i] for s in strs)). This ensures that including the character at column i after j maintains the non-decreasing order.

5. If the condition is true, update dp[i] to the maximum of its current value or dp[j] + 1. In other words, extend the length of the ordered subsequence ending at j by one to include i.

6. After filling the dp array, find the length of the longest non-decreasing subsequence by finding the maximum value in dp (max(dp)).

7. Since we need to find the minimum number of deletions, subtract the length of the longest non-decreasing subsequence from the total number of columns, n - max(dp). This gives us the minimum columns that need to be deleted to ensure all strings are in lexicographic order.

The principle algorithms and patterns used in this solution include:

• Dynamic Programming: Through the dp array to store intermediate results and avoid redundant computations.
• Nested Loops: To compare every possible pair of columns.
• Greedy Choice: At each step, choosing the longest subsequence ending at j that can be extended by i.

By incorporating dynamic programming, the solution effectively leverages previously computed results to build upon and find the longest subsequence, reducing the complexity compared to naive approaches that might check every possible combination of deletions.

Example Walkthrough

Let's use a small example to illustrate the solution approach with the given strings array strs = ["cba", "daf", "ghi"].

1. Since each string has three characters, we have n = 3.

2. Initialize the dp array with n elements to [1, 1, 1] because each character alone is a non-decreasing subsequence.

3. We start the nested loop iteration.

   • For i = 1, we compare with j = 0.
     • Compare every string's character at index j with index i.
     • For "cba", "daf", and "ghi", we compare pairs ("c", "b"), ("d", "a"), ("g", "h").
     • Since "c" > "b", "d" > "a", "g" > "h", the condition isn't met and we don't update dp[1].
   • Move to i = 2, again, compare with j = 0 and then j = 1.
     • Comparing index 0 with 2, for "cba" ("c", "a"), "daf" ("d", "f"), and "ghi" ("g", "i"), it's not non-decreasing for all sequences.
     • Now compare index 1 with 2, for "cba" ("b", "a"), "daf" ("a", "f"), and "ghi" ("h", "i"). Only in "ghi" is "h" <= "i". Since not all strings fulfill the condition, dp[2] remains unchanged.

4. At this point, our dp array is still [1, 1, 1] as no increasing subsequences were found that include more than one column.

5. The length of the longest non-decreasing subsequence is simply the max value in dp, which is 1.

6. To find the minimum number of deletions, n - max(dp), we subtract the longest subsequence's length (1) from the total number of columns (3).

Therefore, 3 - 1 = 2 is the minimum number of deletions required. For this example:

• Deleting the first and second columns ("c" from "cba", "d" from "daf", "g" from "ghi" and "b" from "cba", "a" from "daf", "h" from "ghi") leaves us with ["a", "f", "i"], which are in non-decreasing order for each string.

By following the dynamic programming approach, we efficiently found the minimum number of columns to delete without exhaustively checking every combination. This example illustrates the steps and logic of the solution approach clearly.

Solution Implementation

Time and Space Complexity

The provided code performs a dynamic programming approach to find the minimum number of columns that need to be deleted from a list of equal-length strings to ensure that the remaining columns are lexicographically sorted. Here's an analysis of its complexities:

Time Complexity

The time complexity of the code is O(n^2 * m), where n is the length of each string in strs and m is the number of strings. This is because the code uses a nested loop structure where the outer loop runs n times (the number of columns), and the inner loop also runs up to n times for each iteration of the outer loop. Inside the inner loop, there is a comparison that runs m times for checking if every string maintains the lexicographic order for the pair of columns being considered. Multiplying all these together gives O(n^2 * m) as the time complexity.

Space Complexity

The space complexity of the function is O(n), which is due to the dynamic programming array dp of size n, where each element represents the length of the longest subsequence of sorted columns including the current column as the last sorted column. Additional space usage is constant and doesn't scale with input size (n or m), hence the overall space complexity is O(n).

Learn more about how to find time and space complexity quickly using problem constraints.