Levenshtein Distance

Levenshtein distance measures how many single-character edits are needed to turn one string into another.

Allowed edits:

• insert one character
• delete one character
• substitute one character for another

Example:

kitten -> sitten   substitute k with s
sitten -> sittin   substitute e with i
sittin -> sitting  insert g

Distance: 3

Dynamic Programming

Let d[i][j] be the edit distance between:

• the first i characters of string a
• the first j characters of string b

Base cases:

d[0][j] = j
d[i][0] = i

Those mean an empty string needs j insertions to become a prefix of length j, and a prefix of length i needs i deletions to become empty.¹

Recurrence²:

cost = 0 if a[i - 1] == b[j - 1] else 1

d[i][j] = min(
    d[i - 1][j] + 1,        # delete
    d[i][j - 1] + 1,        # insert
    d[i - 1][j - 1] + cost  # substitute or match
)

The answer is d[len(a)][len(b)].

Complexity

For strings of length m and n:

• time: O(mn)
• memory: O(mn) with the full table
• memory: O(min(m, n)) if only the previous row is kept

Use the full table when you need the edit script. Use row compression when you only need the distance.

Use Case

Levenshtein distance is useful for:

• spell correction
• fuzzy search
• typo tolerance
• approximate matching
• comparing biological or symbolic sequences