【leetcode】944. Delete Columns to Make Sorted

题目如下：

We are given an array A of N lowercase letter strings, all of the same length.

Now, we may choose any set of deletion indices, and for each string, we delete all the characters in those indices.

For example, if we have a string "abcdef" and deletion indices {0, 2, 3}, then the final string after deletion is "bef".

Suppose we chose a set of deletion indices D such that after deletions, each remaining column in A is in non-decreasing sorted order.

Formally, the c-th column is [A[0][c], A[1][c], ..., A[A.length-1][c]]

Return the minimum possible value of D.length.

Example 1:

Input: ["cba","daf","ghi"]
Output: 1

Example 2:

Input: ["a","b"]
Output: 0

Example 3:

Input: ["zyx","wvu","tsr"]
Output: 3

Note:

1. 1 <= A.length <= 100
2. 1 <= A[i].length <= 1000

解题思路：如果不考虑时间复杂度，那么这题的级别应该是Easy，而不是Medium。O(n^2)的方法也很简单，遍历每一组序列，如果不满足递增则要删除这一组。

代码如下：

class Solution(object):
    def minDeletionSize(self, A):
        """
        :type A: List[str]
        :rtype: int
        """
        for i in range(len(A)):
            A[i] += '{'
        res = 0
        for j in range(len(A[0])):
            for i in range(len(A)-1):
                if A[i][j] > A[i+1][j]:
                    res += 1
                    break
        return res