Leetcode:permutations Problem Solving Report

Permutations

Given A collection of numbers, return all possible permutations.

For example,
[following] has the permutations:
[1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Solution 1:

Classic recursive backtracking topic, one accept. Please also refer to the previous topic Leetcode:combinations problem Solving report.

1  Public classSolution {2      PublicList<list<integer>> Permute (int[] num) {3list<list<integer>> ret =NewArraylist<list<integer>>();4         if(num = =NULL|| Num.length = = 0) {5             returnret;6         }7         8DFS (NUM,NewArraylist<integer>(), ret);9         returnret;Ten     } One      A      Public voidDfsint[] num, list<integer> path, list<list<integer>>ret) { -         intLen =num.length; -         if(path.size () = =Len) { theRet.add (NewArraylist<integer>(path)); -             return; -         } -          +          for(inti = 0; i < Len; i++) { -             if(Path.contains (Num[i])) { +                 Continue; A             } at              - Path.add (Num[i]); - dfs (num, path, ret); -Path.remove (Path.size ()-1); -         } -     } in}

View Code

Solution 2:

Maybe some students think why path.contains do not hashmap to replace the miles? So the homepage June wrote a version with HashMap. The conclusion is that when the set size is small, the performance of HashMap is not

such as ArrayList.

The reason may be that the HashMap application is not a continuous space, and ArrayList relatively small, directly in continuous memory operation, the speed will be relatively fast.

The following is the result of running this program, and the version of HashMap is one times slower:

Test Size:9
Computing time with hashmap:629.0 millisec.
Test Size:9
Computing time with list:310.0 millisec.

1  Packagealgorithms.permutation;2 3 Importjava.util.ArrayList;4 ImportJava.util.HashSet;5 ImportJava.util.LinkedHashMap;6 7  Public classPermutation {8      Public Static voidMain (string[] strs) {9         int[] num = {1, 2, 3, 4, 5, 6, 7, 8, 9};TenSystem.out.printf ("Test size:%d \ n", num.length); One  AStopwatch timer1 =NewStopwatch (); -          - permute (num); the System.out -. println ("Computing Time with HASHMAP:" -+ timer1.elapsedtime () + "millisec."); -          +System.out.printf ("Test size:%d \ n", num.length); -  +Stopwatch Timer2 =NewStopwatch (); A          at permute2 (num); - System.out -. println ("Computing Time with List:" -+ timer2.elapsedtime () + "millisec."); -     } -      in      Public StaticArraylist<arraylist<integer>> Permute (int[] num) { -arraylist<arraylist<integer>> ret =NewArraylist<arraylist<integer>>(); to         if(num = =NULL) { +             returnret; -         } the          *Permutehelp (NUM, ret,NewLinkedhashmap<integer, integer>()); $         returnret;Panax Notoginseng     } -      the      Public Static voidPermutehelp (int[] num, arraylist<arraylist<integer>> ret, Linkedhashmap<integer, integer>set) { +         if(set.size () = =num.length) { A              thearraylist<integer> list =NewArraylist<integer>(); +              for(Integer i:set.keyset ()) { - List.add (i); $             } $ Ret.add (list); -             return; -         } the          -         intLen =num.length;Wuyi          for(inti = 0; i < Len; i++) { the             if(Set.containskey (Num[i])) { -                 Continue; Wu             } -              About             //Path.add (Num[i]); $Set.put (Num[i], 0); - permutehelp (num, ret, set); -             //Path.remove (Path.size ()-1); - Set.remove (Num[i]); A         } +     } the      -      Public StaticArraylist<arraylist<integer>> Permute2 (int[] num) { $arraylist<arraylist<integer>> ret =NewArraylist<arraylist<integer>>(); the         if(num = =NULL) { the             returnret; the         } the          -arraylist<integer> Path =NewArraylist<integer>(); in permuteHelp2 (num, path, ret); the         returnret; the     } About      the      Public Static voidPERMUTEHELP2 (int[] num, arraylist<integer> path, arraylist<arraylist<integer>>ret) { the         if(path.size () = =num.length) { theRet.add (NewArraylist<integer>(path)); +             return; -         } the         Bayi         intLen =num.length; the          for(inti = 0; i < Len; i++) { the             if(Path.contains (Num[i])) { -                 Continue; -             } the              the Path.add (Num[i]); the permuteHelp2 (num, path, ret); thePath.remove (Path.size ()-1); -         } the     } the}

GITHUB:

Https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/dfs/Permute.java

Leetcode:permutations Problem Solving Report