find valid combination k number sum n follow condition trueonly number 1 9 usedeach number oncereturn list possible valid combination the list contain combination twice combination return order Example 1input k 3 n 7output 124explanation1 2 4 7there valid combinationsexample 2input k 3 n 9output 126135234explanation1 2 6 91 3 5 92 3 4 9there valid combinationsExample 3input k 4 n 1output Explanation there valid combinationsuse 4 different number range 19 small sum 1234 10 10 1 valid combination Constraints2 k 91 n 60