Delete Columns to Make Sorted II — LeetCode 955 Python Solution

The problem

You are given an array of n strings strs, all of the same length. We may choose any deletion indices, and we delete all the characters in those indices for each string.

Example

Input

strs = ["ca","bb","ac"]

Output

1

Explanation

After deleting the first column, strs = ["a", "b", "c"].

Python solution

class Solution:
    def minDeletionSize(self, strs: List[str]) -> int:
        n = len(strs)
        m = len(strs[0])
        st = [False] * (n - 1)
        ans = 0
        for j in range(m):
            must_del = False
            for i in range(n - 1):
                if not st[i] and strs[i][j] > strs[i + 1][j]:
                    must_del = True
                    break
            if must_del:
                ans += 1
            else:
                for i in range(n - 1):
                    if not st[i] and strs[i][j] < strs[i + 1][j]:
                        st[i] = True
        return ans

Complexity

Pattern: Greedy

Take the locally best option every time — when you can prove that never costs you later. LeetCode 955. Delete Columns to Make Sorted II is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Greedy.

The greedy guide has the Python template for the pattern and the 346 LeetCode problems that use it.

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