distance between two strings

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The edit distance between two strings refers to the minimum number of character insertions, deletions, and substitutions required to change one string to the other. For example,
Given two strings, compute the edit distance between them.

Write a function solve that have the following parameter(s):
1.) a = a string
2.) b = a string

Example
Input:
a= kitten
b= sitting
Output:
3

Explanation
The edit distance between kitten and sitting is 3:
substitute the k for s
substitute the e for i
append a g

Constraints
• The size of a string should be greater than 0 and less than 100.