Create diff_two_strings.md

Author: Joseph Luce

real_interview_questions/Other/diff_two_strings.md

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+# Diff Between Two Strings
+
+Given two strings of uppercase letters source and target, list (in string form) a sequence of edits to convert from source to target that uses the least edits possible.
+
+For example, with strings source = "ABCDEFG", and target = "ABDFFGH" we might return: ["A", "B", "-C", "D", "-E", "F", "+F", "G", "+H"
+
+More formally, for each character C in source, we will either write the token C, which does not count as an edit; or write the token -C, which counts as an edit.
+
+Additionally, between any token that we write, we may write +D where D is any letter, which counts as an edit.
+
+At the end, when reading the tokens from left to right, and not including tokens prefixed with a minus-sign, the letters should spell out target (when ignoring plus-signs.)
+
+In the example, the answer of A B -C D -E F +F G +H has total number of edits 4 (the minimum possible), and ignoring subtraction-tokens, spells out A, B, D, F, +F, G, +H which represents the string target.
+
+If there are multiple answers, use the answer that favors removing from the source first.
+
+Constraints:
+
+[time limit] 5000ms
+
+[input] string source
+2 ≤ source.length ≤ 12
+
+[input] string target
+2 ≤ target.length ≤ 12
+
+[output] array.string
+
+# Solution
+
+The trickiest part of all this is to build the intuition for how to handle the fork.
+You should notice that there are only two possible ways when a char from target and source do not match.
+Whether to delete S or add T.
+We would have to traverse the entire solution space because we don't know if we can build a smaller list.
+After that, its a simpler recursive call and returning the min list.
+
+```
+def diffBetweenTwoStrings(source, target):
+  def diff_helper(s, t):
+    if len(t) == 0 and len(s) > 0:
+      return ['-' + ch for ch in s]
+    elif len(t) > 0 and len(s) == 0:
+      return ['+' + ch for ch in t]
+    elif len(t) == 0 and len(s) == 0:
+      return []
+    if s[0] == t[0]:
+      return [s[0]] + diff_helper(s[1:], t[1:])
+    # s[0] != t[0]
+    result1 = diff_helper(s[1:], t) # skip s, delete s
+    result2 = diff_helper(s, t[1:]) # skip t, add t
+    if len(result1) <= len(result2):
+      return ['-' + s[0]] + result1
+    else:
+      return ['+' + t[0]] + result2
+    
+  return diff_helper(source, target)
+```