notes/OJ notes/pages/Leetcode Reverse-String.md

# Leetcode Reverse String

#### 2022-07-12 08:50

> ##### Algorithms:
>
> #algorithm #recursion
>
> ##### Data structures:
>
> #DS #array
>
> ##### Difficulty:
>
> #coding_problem #difficulty_easy
>
> ##### Additional tags:
>
> #leetcode
>
> ##### Revisions:
>
> N/A

##### Related topics:

##### Links:

- [Link to problem](https://leetcode.com/problems/reverse-string/)

---

### Problem

Write a function that reverses a string. The input string is given as an array of characters `s`.

You must do this by modifying the input array [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) with `O(1)` extra memory.

#### Examples

**Example 1:**

**Input:** s = ["h","e","l","l","o"]
**Output:** ["o","l","l","e","h"]

**Example 2:**

**Input:** s = ["H","a","n","n","a","h"]
**Output:** ["h","a","n","n","a","H"]

#### Constraints

- `1 <= s.length <= 105`
- `s[i]` is a [printable ascii character](https://en.wikipedia.org/wiki/ASCII#Printable_characters).

### Thoughts

> [!summary]
> This is a #recursion problem, also can be solved using iteration.

#### Base case:

- size <= 1

#### Pseudo code:

- swap two ends
- recurse the sub string(without the ends)

### Solution

Iteration

```cpp
class Solution {
public:
  void reverseString(vector<char> &s) {
    int l = 0, r = s.size() - 1;

    while (l < r) {
      swap(s[l], s[r]);

      l++;
      r--;
    }
  }
};
```

Recursion

```cpp
class Solution {
  void reverse(vector<char> &s, int l, int r) {
    if (l >= r) {
      return;
    }

    swap(s[l++], s[r--]);
    reverse(s, l, r);
  }

public:
  void reverseString(vector<char> &s) { reverse(s, 0, s.size() - 1); }
};
```