# Principle of Recursion

1. A simple `base case` (or cases) — a terminating scenario that does not use recursion to produce an answer.
2. A set of rules, also known as `recurrence relation` that reduces all other cases towards the base case.

344. Reverse String

https://leetcode.com/problems/reverse-string/

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"]`

24. Swap Nodes in Pairs

https://leetcode.com/problems/swap-nodes-in-pairs

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list’s nodes. Only nodes itself may be changed.

Example 1:

`Input: head = [1,2,3,4]Output: [2,1,4,3]`

Example 2:

`Input: head = []Output: []`

Example 3:

`Input: head = Output: `

Constraints:

• The number of nodes in the list is in the range `[0, 100]`.
• `0 <= Node.val <= 100`

Practice how to convert For Loop to RECURSION

Sum- make a sum of string composed with numbers

ex. ‘123’

# Recurrence Relation

• `recurrence relation`: the relationship between the result of a problem and the result of its subproblems.
• `base case`: the case where one can compute the answer directly without any further recursion calls. Sometimes, the base cases are also called bottom cases, since they are often the cases where the problem has been reduced to the minimal scale, i.e. the bottom, if we consider that dividing the problem into subproblems is in a top-down manner.

206. Reverse Linked List

Reverse a singly linked list.

Example:

`Input: 1->2->3->4->5->NULLOutput: 5->4->3->2->1->NULL`