Single letter swaps
Write a function that takes a sequence of strings and a target string. For each string in the sequence, determine if it is equal to the target string after exactly one letter swap. Return the sequence of letter pairs that are swapped, or nil if it doesn't exist.
Example
(letter-swaps ["bacd" "abdc" "abcde" "abcc" "abcd"] "abcd")
;=> [#{\a \b} #{\c \d} nil nil nil]
Swapping a and b in "bacd" gives you the target string. Swapping c and d in "abdc" gives you the target string. But there is no way to swap to get an extra e. And trading a d for a c is not possible. Finally, the last string has no swaps, and exactly one is required.
Thanks to this site for the problem idea, where it is rated Very Hard in PHP. The problem has been modified.
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Here are a couple of additional examples that I think should be true, but not all the implementations agree.