“This is the third day of my participation in the November Gwen Challenge. See details of the event: The last Gwen Challenge 2021”.

Flip the string

1. Title Description

Write a function that reverses the input string. The input string is given in the form of the character array char[].

Instead of assigning extra space to another array, you must modify the input array to use O(1) extra space to solve the problem.

You can assume that all the characters in the array are printable characters in the code table.

Example 1:

Input:"h"."e"."l"."l"."o"] output: ["o"."l"."l"."e"."h"]
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Example 2:

Input:"H"."a"."n"."n"."a"."h"] output: ["h"."a"."n"."n"."a"."H"]
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2. How to solve the problem

1. Built-in Slices in Python

   Do this using the slice and step size of a list in Python

2. Double pointer method

   Use double Pointers

   A header pointer

   A tail pointer

   Swaps the values of the elements corresponding to the header and tail Pointers

   Know that the head pointer is smaller than the tail pointer

Three, code,

1. slice

   class Solution:
       def reverseString(self, s: List[str]) - >None:
           """ Do not return anything, modify s in-place instead. """
           s[:] = s[::-1]
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2. Double pointer

   class Solution:
       def reverseString(self, s: List[str]) - >None:
           """ Do not return anything, modify s in-place instead. """
           start = 0
           end = len(s)-1
           while start < end:
               mid = s[start]
               s[start] = s[end]
               s[end] = mid
               start += 1
               end -= 1
           
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Second,

1. Title Description

Given a string, you need to reverse the character order of each word in the string, while still preserving the Spaces and the original order of the words.

Example:

Enter: “Let’s take LeetCode contest” Output: “s ‘tel ekat edoCteeL tsetnoc”

Tip:

In a string, each word is separated by a single space, and there are no extra Spaces in the string.

Second, the topic analysis

1. The Python built-in function method

   • It doesn’t say it’s in the same place

   • So we split the string with a space (split())

   • I have a list of strings

   • Then reverse order the list elements (slice)

   • Finally, convert the list to a string (join())

2. Double pointer method

   • Iterate over the string
   • Stop on whitespace
   • And now I have the space subscript (I)
   • Subscript j= 0 and then the space subscript I
   • I loop over j and I -1 until j < I
   • And then the next word
   • The end of the

Three, code,

1. Python built-in functions

   class Solution:
       def reverseWords(self, s: str) - >str:
           s_li = s.split(' ')
           for i in range(len(s_li)):
               s_li[i] = s_li[i][::-1]
               print(s_li[i])
           return ' '.join(s_li)
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2. Double pointer

   class Solution:
       def reverseWords(self, s: str) - >str:
           sli = list(s) # Convert a string to a list
           m = 0
           x = len(s)-1
           for i in range(len(sli)):
               if sli[i] == ' ':
                   n = i-1
                   while m < n:
                       mid = s[m]
                       sli[m] = sli[n]
                       sli[n] = mid
                       m += 1
                       n -= 1
                   m = i+1
           while m < x:  # There is no space after the last word in the string
               mid = s[m]
               sli[m] = sli[x]
               sli[x] = mid
               m += 1
               x -= 1
           return ' '.join(sli)
   
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3. Double pointer II

   class Solution:
       def reverseWords(self, s: str) - >str:
           sli = list(s)
           m = 0
           sli.append("")
           for i in range(len(sli)):
               if sli[i] == ' ':
                   n = i-1
                   while m < n:
                       mid = s[m]
                       sli[m] = sli[n]
                       sli[n] = mid
                       m += 1
                       n -= 1
                   m = i+1
           sli[len(sli)-1] = ' '
           return ' '.join(sli)
   
           
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   Add a space and then delete it