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Learning Opportunities

This puzzle can be solved using the following concepts. Practice using these concepts and improve your skills.

Statement

 Goal

You have to solve a 25x25 sudoku grid!

An empty cell is represented by . and the letters A to Y are used to fill the grid.

RULES:
Each line contains exactly one occurrence of each letter.
Each column contains exactly one occurrence of each letter.
Each block (5x5 cells) contains exactly one occurrence of each letter.

NOTE: This puzzle is considerably more difficult than a classic sudoku puzzle like https://www.codingame.com/training/medium/sudoku-solver. Simple backtracking will not be enough, knowing that there can be more than 300 unrevealed cells.
Input
Line 1 to Line 25 : 25 characters in the range A..Y or . (empty cell)
Output
Line 1 to Line 25 : 25 characters in the range A..Y
Constraints
Every grid has a unique solution.
Example
Input
.FLJ...D..O.E..GM.KSNRQXA
...HK.CXROUV...QF.BESJP..
RNMY.BQETA.F...XOVP.W.HD.
QO..PMK.SYN.IAR.L.C....EB
.E...L..G.BPHQD.RN.TYO...
YT..JI..UVG..BL.E.XOCM..R
HS....A.KLC..O..PF.W.I.VY
X.FR...OQ.IKWTPYD.U.A.EH.
N....PYSFX.UQ..BVGHC.KJO.
...LC...WDH..VM.T.I....NX
...B..IYH.DLAX..SKW.O....
..HP..NA.....IQTGU..XYSWE
K.O.YGSQDBW......XE..LMRP
UQNWR.PTLEY.O.VAH...KCI..
T..XDOW....E.PN..I.LGU.BH
..R.NDGC....MEJUKOS...FYW
..GDH..R..V..NCL.M.QE.XJ.
..J.F....QAW....XR.PMHO.I
.U.S.JEK.N.R.LI.WTYBV..A.
VW....OL...HDK.....I.NRQU
....T.BHN.JDCW.O.EL...V..
.L..BFX.AGP...KM..VH..T..
...O.Y..E..NVFB...TX..LK.
E.K...LI...S...F...AD.YG.
DXYVQ.JPOT.ML.A.ISNGH.B.F
Output
BFLJUHVDPIOCEYTGMWKSNRQXA
ADTHKNCXROUVGMWQFYBESJPIL
RNMYIBQETALFKJSXOVPUWGHDC
QOVGPMKWSYNXIARDLHCJTFUEB
SEXCWLFJGUBPHQDIRNATYOKMV
YTDKJIHNUVGAFBLSEQXOCMWPR
HSBQETAGKLCJNOXRPFMWUIDVY
XGFRVCMOQJIKWTPYDLUNABEHS
NIWMAPYSFXRUQDEBVGHCLKJOT
OPULCERBWDHYSVMJTAIKFQGNX
JCEBGUIYHMDLAXFPSKWROVNTQ
FMHPLVNACRKBJIQTGUODXYSWE
KAOIYGSQDBWTUCHVNXEFJLMRP
UQNWRXPTLEYGOSVAHBJMKCIFD
TVSXDOWFJKMERPNCYIQLGUABH
LBRANDGCIHXQMEJUKOSVPTFYW
IYGDHWURBPVOTNCLAMFQESXJK
CKJEFSTVYQAWBGUNXRDPMHOLI
MUQSOJEKXNFRPLIHWTYBVDCAG
VWPTXAOLMFSHDKYECJGIBNRQU
PRAFTKBHNSJDCWGOQELYIXVUM
WLCNBFXUAGPIYRKMJDVHQETSO
GHIOSYDMECQNVFBWUPTXRALKJ
EJKUMQLIVWTSXHOFBCRADPYGN
DXYVQRJPOTEMLUAKISNGHWBCF

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