Given a collection of numbers, return all possible permutations.
For example,[1,2,3] have the following permutations:[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].
1 public class Solution { 2 ArrayList<ArrayList<Integer>> result = null; 3 int len = 0; 4 public ArrayList<ArrayList<Integer>> permute(int[] num) { 5 // Start typing your Java solution below 6 // DO NOT write main() function 7 result = new ArrayList<ArrayList<Integer>>(); 8 if(num == null || num.length == 0) return result; 9 len = num.length; 10 boolean[] item = new boolean[len]; 11 get(item, num, new ArrayList<Integer>()); 12 return result; 13 } 14 public void get(boolean[] item, int[] num, ArrayList<Integer> row){ 15 if(row.size() == len){ 16 result.add(new ArrayList<Integer>(row)); 17 return; 18 } 19 for(int i = 0; i < len; i ++){ 20 if(!item[i]){ 21 item[i] = true; 22 row.add(num[i]); 23 get(item, num, row); 24 item[i] = false; 25 row.remove(row.size() - 1); 26 } 27 } 28 } 29 }
第三遍：
1 public class Solution { 2 public List<List<Integer>> permute(int[] num) { 3 ArrayList<List<Integer>> result = new ArrayList<List<Integer>>(); 4 getPermutation(num, num.length, new ArrayList<Integer>(), result); 5 return result; 6 } 7 public void getPermutation(int[] num, int len, ArrayList<Integer> row, ArrayList<List<Integer>> result){ 8 if(num == null || len == 0){ 9 result.add(row); 10 return; 11 } 12 for(int i = 0; i < len; i ++){ 13 ArrayList<Integer> nrow = new ArrayList<Integer>(row); 14 nrow.add(num[i]); 15 int tmp = num[i]; 16 num[i] = num[len - 1]; 17 getPermutation(num, len - 1, nrow, result); 18 num[i] = tmp; 19 } 20 } 21 }