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README.md

Levenshtein distance

This is a well known problem.

Illustrating this problem using wikipedia's example

Making a table like Interleaving String

+---+---+---+---+---+---+---+---+
|   | * | k | i | t | t | e | n |
+---+---+---+---+---+---+---+---+
| * |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| s |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| n |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| g |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+

* here is representing an empty string

each gird is the edit distance of string[0..x] and string[0..y]

First row and column girds are easy to be filled:

  • edit distance of an empty string to an empty string is 0
  • edit distance of an empty string to any string is the length of that string

So the table should be filled like:

+---+---+---+---+---+---+---+---+
|   | * | k | i | t | t | e | n |
+---+---+---+---+---+---+---+---+
| * | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+---+---+---+---+---+---+---+---+
| s | 1 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i | 2 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 3 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 4 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i | 5 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| n | 6 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| g | 7 |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+

what's about k and s

in k's view

  • Insert: cant transform to s
  • Delete: cant transform to s
  • Replace: replace k to s use 1 edit distance

in s's view

  • Insert: cant transform to k
  • Delete: cant transform to k
  • Replace: replace s to k use 1 edit distance

so the number in the gird k and s should be 1

what's about ki and s (in ki's view)

  • Insert: cant transform to s
  • Delete: delete i then, transform to the problem s and k we solved before. 1 + edit distance of s and k.
  • Replace and Delete: replace k to s use 1 edit distance, then delete k use 1 edit distance. 1 + 1 = 2

Delete and Replace then Delete are the same thing at last, Replace is just the 1 edit distance costed by edit distance of s and k.

In this case, we can transform the problem by deleting one char: 1 + edit distance of string deleted one and k

We can fill up the second column and row now:

+---+---+---+---+---+---+---+---+
|   | * | k | i | t | t | e | n |
+---+---+---+---+---+---+---+---+
| * | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+---+---+---+---+---+---+---+---+
| s | 1 | 1 | 2 | 3 | 4 | 5 | 6 |
+---+---+---+---+---+---+---+---+
| i | 2 | 2 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 3 | 3 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 4 | 4 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i | 5 | 5 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| n | 6 | 6 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| g | 7 | 7 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+

What if si and ki

Obviously, si and ki equals to the edit distance of s and k, because i are both in two strings.

+---+---+---+---+---+---+---+---+
|   | * | k | i | t | t | e | n |
+---+---+---+---+---+---+---+---+
| * | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+---+---+---+---+---+---+---+---+
| s | 1 | 1 | 2 | 3 | 4 | 5 | 6 |
+---+---+---+---+---+---+---+---+
| i | 2 | 2 | 1 |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 3 | 3 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| t | 4 | 4 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| i | 5 | 5 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| n | 6 | 6 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| g | 7 | 7 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+

then we can fill up the table using technology we used before.

More common case

P[word1][word2] is the the table,

so P[i][j] have 4 choices:

  • P[i - 1][j] + 1 : by deleting word1[i] from word1 and convert to previous problem.
  • P[i][j - 1] + 1 : by deleting word2[j] from word2 and convert to previous problem.
  • P[i - 1][j - 1] : only if word1[i] == word2[j]
  • P[i - 1][j - 1] + 1 : only if word1[i] != word2[j], then replace one char word1[i] to word2[j], then this problem is converted to P[i - 1][j - 1].

Dont worry about Insertion, it is just the oppsite to Deletion. Just different view.

So the work remaing is easy, find the MIN of those four is ok.