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Added questions 17 and 39 in Java

Added question 17. Letter Combinations of a Phone Number and 39. Combination Sum in Java

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	`1`	`+importjava.util.ArrayList;`
	`2`	`+importjava.util.List;`
	`3`	`+importjava.util.Map;`
	`4`	`+`
	`5`	`+classSolution {`
	`6`	`+privateMap<Character,String>digitToChar =Map.of(`
	`7`	`+'2',"abc",'3',"def",'4',"ghi",'5',"jkl",`
	`8`	`+'6',"mno",'7',"pqrs",'8',"tuv",'9',"wxyz");`
	`9`	`+`
	`10`	`+publicList<String>letterCombinations(Stringdigits) {`
	`11`	`+`
	`12`	`+if (digits.length() ==0) {`
	`13`	`+returnnewArrayList();`
	`14`	`+ }`
	`15`	`+`
	`16`	`+List<String>ans =newArrayList();`
	`17`	`+Stringcur ="";`
	`18`	`+backtrack(digits,ans,cur,0);`
	`19`	`+returnans;`
	`20`	`+`
	`21`	`+ }`
	`22`	`+`
	`23`	`+publicvoidbacktrack(Stringdigits,List<String>ans,Stringcur,intindex) {`
	`24`	`+`
	`25`	`+if (cur.length() ==digits.length()) {`
	`26`	`+ans.add(cur);`
	`27`	`+return;`
	`28`	`+ }elseif (index >=digits.length()) {`
	`29`	`+return;`
	`30`	`+ }else {`
	`31`	`+Stringdigit =digitToChar.get(digits.charAt(index));`
	`32`	`+for (charc :digit.toCharArray()) {`
	`33`	`+backtrack(digits,ans,cur +c,index +1);`
	`34`	`+ }`
	`35`	`+ }`
	`36`	`+ }`
	`37`	`+}`

Lines changed: 24 additions & 0 deletions

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	`1`	`+classSolution {`
	`2`	`+publicList<List<Integer>>combinationSum(int[]candidates,inttarget) {`
	`3`	`+List<List<Integer>>ans =newArrayList<List<Integer>>();`
	`4`	`+List<Integer>cur =newArrayList();`
	`5`	`+backtrack(candidates,target,ans,cur,0);`
	`6`	`+returnans;`
	`7`	`+`
	`8`	`+ }`
	`9`	`+`
	`10`	`+publicvoidbacktrack(int[]candidates,inttarget,List<List<Integer>>ans,List<Integer>cur,intindex) {`
	`11`	`+if (target ==0) {`
	`12`	`+ans.add(newArrayList(cur));`
	`13`	`+ }elseif (target <0 \|\|index >=candidates.length) {`
	`14`	`+return;`
	`15`	`+ }else {`
	`16`	`+cur.add(candidates[index]);`
	`17`	`+backtrack(candidates,target -candidates[index],ans,cur,index);`
	`18`	`+`
	`19`	`+cur.remove(cur.get(cur.size() -1));`
	`20`	`+backtrack(candidates,target,ans,cur,index +1);`
	`21`	`+ }`
	`22`	`+`
	`23`	`+ }`
	`24`	`+}`

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