ctci/17. Hard problems/17.14. word_transformer.md

17.14. Word transformer

Given two words of equal lengths that are in the dictionary, transform one word into another word by changing only one letter at a time. The new word you get in each step must be in the dictionary

I guess we have to return the number of steps?

example: input: "DAMP", "LIKE" "DAMP" -> "LAMP" -> "LIMP" -> "LIME" -> "LIKE"

Initial scenario

Only len(s) changes must be done. Iterate through s1, see if s1[:i] + s2[i] + s1[i:] is present in dct, if so change and update boolean array. If not, i+1.

dct = ["damp", "lamp", "limp", "lime", "like"]

def transform(s1, s2):

    if len(s1) != len(s2):
        return -1

    n = len(s1)
    found = [0] * n
    steps = 0
    for i in range(n):
        pos_s = s1[:i] + s2[i] + s1[i+1:]
        if pos_s in dct and not found[i]:
            s1 = pos_s
            found[i] = 1
            steps += 1

    for i in range(n):
        if found[i]:
            continue
        pos_s = s1[:i] + s2[i] + s1[i+1:]
        if pos_s in dct:
            s1 = pos_s
            found[i] = 1
            steps += 1

    return steps

This takes O(2n) time and O(n) space. Not bad for a start, let's think of more complicated examples.

More complicated examples

What if we need a temporary random letter not present in s1 or s2? No direct transformations?

"damp" -> "kamp" -> "kalp" -> "malp"

The only transformation from "mamp" is "kamp", which doesn't necessarily bring us closer to our second string. This looks like a graph/shortest path algorithm, no idea how to tackle this as of 9 may 2019.