Beaten by minutes!
/* The input suduko. */
WITH RECURSIVE input(sud) AS (
VALUES(
'53 7 6 195 98 6 8 6 34 8 3 17 2 6 6 28
419 5 8 79'
)
),
/* A table filled with digits 1..9, inclusive. */
digits(z, lp) AS (
VALUES('1', 1)
UNION ALL SELECT
CAST(lp+1 AS TEXT), lp+1 FROM digits WHERE lp<9
),
/* The tricky bit. */
x(s, ind) AS (
SELECT sud, instr(sud, ' ') FROM input
UNION ALL
SELECT
substr(s, 1, ind-1) || z || substr(s, ind+1),
instr( substr(s, 1, ind-1) || z || substr(s, ind+1), ' ' )
FROM x, digits AS z
WHERE ind>0
AND NOT EXISTS (
SELECT 1 FROM digits AS lp
WHERE z.z = substr(s, ((ind-1)/9)*9 + lp, 1)
OR z.z = substr(s, ((ind-1)%9) + (lp-1)*9 + 1, 1)
OR z.z = substr(s, (((ind-1)/3) % 3) * 3
+ ((ind-1)/27) * 27 + lp
+ ((lp-1) / 3) * 6
, 1)
)
)
SELECT s FROM x WHERE ind=0;
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