# Leetcode - Episode 4 - gnivloS melborP (3x E)

January 04, 2019 in Algorithms

Two out of three problems involve reversing today hence the title.

It’s also warmer than yesterday and my cold has passed .. onto to my fiancée (sorry).

### 557. Reverse Words in a String III

Problem: Reverse the words of a string `N`

while preserving the word order.

`'Apple two' => 'owt elppA'`

This solution came to me pretty quickly. I was able to use some ideas from previous problems that were fresh in my head.

```
class Solution:
def reverseWords(self, s):
"""
:type s: str
:rtype: str
"""
return ' '.join(reversed(s[::-1].split(' ')))
```

Runtime complexity: `O(3n)`

-> `O(n)`

.

Space complexity: I’m not sure. Depending on the implementation, I *believe* that it will mimic the runtime complexity.

### 922. Sort Array By Parity II

Problem: Sort array `N`

so that the even indices have even numbers and the odd indices have odd numbers.

This is the next version of the first problem I solved yesterday.

```
class Solution:
def sortArrayByParityII(self, A):
"""
:type A: List[int]
:rtype: List[int]
"""
even = 0
odd = 1
ans = [0] * len(A)
for i in A:
if i % 2 == 0:
ans[even] = i
even += 2
else:
ans[odd] = i
odd += 2
return ans
```

Although it didn’t feel too bad writing this one out, I knew it was sub-optimal and could be done without that extra list.

Runtime complexity is `O(n)`

as its just one pass through.

Space complexity is `O(2n)`

which becomes `O(n)`

.

I looked up some solutions where it was solved in-place without additional data structures so that in future problems I’ll have a better idea of where to head. They involved managing two pointers for even and odd and being lazy about swapping. In my solution, a perfectly sorted list (to the problem spec) would be resorted regardless.

### 344. Reverse String

Problem: Reverse a string `N`

I’ll be kicking myself later on that I chose to solve these easy ones now — assuming I can stick to my three per day target.

```
class Solution:
def reverseString(self, s):
"""
:type s: str
:rtype: str
"""
return s[::-1]
```

Linear complexity all around. Thank you Python for your terseness.

See you tomorrow.

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