247.Strobogrammatic number II

    /** 247.Strobogrammatic Number II * 2016-6-18 by Mingyang * A strobogrammatic number are a number that looks     The same when rotated-degrees (looked at upside).     * Find All strobogrammatic numbers that is is of length = N.     * For example, Given n = 2, return ["11", "69", "88", "96"]. * Using the concept of backtracking, is actually DFS * very good a backtracking topic, because here need to distinguish our odd even number, so we need to separate to discuss * key point 1: * for (char C:MAP.K     Eyset ()) * Key point 2: * Sb.insert (0, C);     * Sb.append (Map.get (c));     * Sb.deletecharat (0);      * Sb.deletecharat (Sb.length ()-1); */      PublicList<string> FindStrobogrammatic1 (intN) {if(N < 1)return NewArraylist<string>(); List<String> res =NewArraylist<string>(); Map<character, character> map =NewHashmap<character, character>(); Map.put (' 0 ', ' 0 '); Map.put (' 1 ', ' 1 '); Map.put (' 6 ', ' 9 '); Map.put (' 8 ', ' 8 '); Map.put (' 9 ', ' 6 '); StringBuilder SB=NewStringBuilder (); intPosition = (n% 2 = = 0)? 0:1;                    DFS (res, SB, map, n, position); returnRes; }            Private voidDFS (list<string> res, StringBuilder SB, Map<character, character> Map,intNintposition) {            if(Sb.length () >N)return; if(sb.length () = =N) {res.add (sb.tostring ()); return; }                  if(Position = = 1) {                 for(CharC:map.keyset ()) {                    if(c = = ' 6 ' | | c = = ' 9 ')                        Continue;                    Sb.append (c); DFS (res, SB, map, n, Position+ 1); Sb.setlength (0); }            } Else {                 for(CharC:map.keyset ()) {                    if(n-sb.length () = = 2 && c = = ' 0 ')                        Continue; Sb.insert (0, c);                    Sb.append (Map.get (c)); DFS (res, SB, map, n, Position+ 2); Sb.deletecharat (0); Sb.deletecharat (Sb.length ()-1); }                }        }     //improved code on-line:       PublicList<string> Findstrobogrammatic (intN) {Map<character, character> map =NewHashmap<character, character>(); Map.put (' 0 ', ' 0 '); Map.put (' 1 ', ' 1 '); Map.put (' 6 ', ' 9 '); Map.put (' 8 ', ' 8 '); Map.put (' 9 ', ' 6 '); List<String> result =NewArraylist<string>(); Char[] buffer =New Char[n]; DFS (N,0, buffer, result, map); returnresult; }       Private voidDfsintNintIndexChar[] buffer, list<string> result, Map<character, character>map) {            if(n = = 0) {                return; }            if(Index = = (n + 1)/2) {Result.add (string.valueof (buffer)); return; }             for(Character c:map.keyset ()) {if(index = = 0 && n > 1 && c = = ' 0 ') {//First Digit cannot is ' 0 ' when n > 1                    Continue; }                if(Index = = N/2 && (c = = ' 6 ' | | c = = ' 9 ')) {//mid digit cannot be ' 6 ' or ' 9 ' when n is odd                    Continue; } Buffer[index]=C; Buffer[n-1-index] =Map.get (c); DFS (n, index+ 1, buffer, result, map); }       } 

247.Strobogrammatic number II