Background
This problem statement is a part of Leetcode's learning card Array and Strings. Under the sub-heading two pointer technique.
Problem Statement
Write a function that reverses a string. The input string is given as an array of characters char[].
Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.
You may assume all the characters consist of printable ASCII characters.
Example 1
Input: ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]
Example 2
Input: ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]
Solution Approach 1
Use python built-in methods. There are two ways of doing the same. Suppose s in the initial list. Then s.reverse() could be done or s[::-1] can be done.
Had reaching the solution by using the built-in methods been the aim. Probably the whole exercise would not have been worth the effort.
Solution Approach 2
- Iterate over the list.
- Swap the first element with the last element.
- The list would be completely reversed when we are at the mid of the list.
- When the index of the current element is equal to mid of the list. Break.
class Solution:
def reverseString(self, s: List[str]) -> None:
"""
Do not return anything, modify s in-place instead.
"""
for x in range(0, len(s)):
if x == int(len(s)/2):
break
else:
temp = s[x]
s[x] = s[len(s)-x-1]
s[len(s)-x-1]=temp
Learnings
- Time complexity - O(n), Space complexity - O(1).
A more elegant approach
Solution approach 3
- Keep two variables. One at the starting of the list at index 0. Another at the end of the list. Index len(L)-1.
- Keep swapping the elements until both the variables reach the mid of the list.
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:
temp = s[start]
s[start] = s[end]
s[end] = temp
start += 1
end -= 1
Learnings
- The solution is still time complexity O(n) and space complexity O(1).
- Two pointer technique. Though in python there is no concept of pointer as such.
- Built-in reverse method is also implemented the same way.
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