public class SummationPuzzle $\{$

public static void main$($String$[]$ args$)\{$

solvePuzzle$()$;

$\}$

static void solvePuzzle$()\{$

int$[]$ digits $=\{7,8,9\}$;

findSolution$($digits$,$ new int$[3],$ new boolean$[3],0)$;

$\}$

static void findSolution$($int$[]$ digits, int$[]$ result, boolean$[]$ used, int index$)\{$

if $($index $==3)\{$

$//$ All digits are assigned, check the equation

int a $=$ result$[0]$;

int b $=$ result$[1]$;

int c $=$ result$[2]$;

$//$ Compute cbb $+$ ba

int cbb $=$ c $*100+$ b $*10+$ b; $//$ cbb

int ba $=$ b $*10+$ a; $//$ ba

int abc $=$ a $*100+$ b $*10+$ c; $//$ abc

if $($cbb $+$ ba $==$ abc$)\{$

System.out.println$("$Solution found: a $="+$ a $+",$ b $="+$ b $+",$ c $="+$ c$)$;

$\}$

return;

$\}$

$//$ Try each digit in the current position

for $($int i $=0$; i $<$ digits.length; i$++)\{$

if $(!$used$[$i$])\{$

used$[$i$]=$ true; $//$ Mark this digit as used

result$[$index$]=$ digits$[$i$]$; $//$ Assign this digit to the result

$//$ Recur to the next index

findSolution$($digits$,$ result, used, index $+1)$;

$//$ Backtrack

used$[$i$]=$ false; $//$ Unmark this digit

$\}$

$\}$

$\}$

$\}$ explain this code, especially how the i and index value change