[LeetCode]#1370. Increasing Decreasing String

Environment: Python 3.7

Key technique: lambda

Given a string s. You should re-order the string using the following algorithm:

1. Pick the smallest character from s and append it to the result.
2. Pick the smallest character from s which is greater than the last appended character to the result and append it.
3. Repeat step 2 until you cannot pick more characters.
4. Pick the largest character from s and append it to the result.
5. Pick the largest character from s which is smaller than the last appended character to the result and append it.
6. Repeat step 5 until you cannot pick more characters.
7. Repeat the steps from 1 to 6 until you pick all characters from s.

In each step, If the smallest or the largest character appears more than once you can choose any occurrence and append it to the result.

Return the result string after sorting s with this algorithm.

Example 1:

Input: s = "aaaabbbbcccc"
Output: "abccbaabccba"

Analysis:

1. Convert string to list
2. use temp.sort(key=lambda c: ord(c)) to sort string according ord(string)
3. ord(‘a’)=97,ord(‘b’)=98…
4. So we can get [‘a’,’b’,’c’]
5. add ‘a’,’b’,’c’ to ans list and remove them from input list
6. use temp.sort(key=lambda c: ord(c), reverse=True) to get [‘c’,’b’,’a’]
7. add ‘c’,’b’,’a’ to ans list and remove them from input list.
8. When s string are removed, return answer.

Solution:

class Solution:
    def sortString(self, s):                
        s = list(s)
        ans = []
        while s:
            temp = list(set(s))  
            temp.sort(key=lambda c: ord(c))  
            for i in temp:
                ans.append(i)
                s.remove(i)
            temp = list(set(s))
            temp.sort(key=lambda c: ord(c), reverse=True)
            for i in temp:
                ans.append(i)
                s.remove(i)
        return ''.join(ans)

Submissions:

Lesson learn:

Below code can sort letter as a,b,c,d….

temp.sort(key=lambda c: ord(c))

Reference:

LeetCode | 1370. Increasing Decreasing String上升下降字符串【Python】

https://ithelp.ithome.com.tw/articles/10218710