17.14. word transformer

looks like a graph / shortest path problem, moving on...
to chapter 2 linked lists

17. Hard problems/17.14. word_transformer.md

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+# 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`.
+
+```python
+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 "**m**amp" 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.

17. Hard problems/17.14. word_transformer.py

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+import unittest
+
+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
+
+class Test(unittest.TestCase):
+    data = [(('damp', 'like'), 4)]
+
+    def test_rotate_matrix(self):
+        for test_num, expected in self.data:
+            actual = transform(*test_num)
+            self.assertEqual(actual, expected)
+
+if __name__ == "__main__":
+    unittest.main()