Permutations

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].

public class Solution {
    ArrayList<ArrayList<Integer>> result = null;
    int len = 0;
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        result = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.length == 0) return result;
        len = num.length;
        boolean[] item = new boolean[len];
        get(item, num, new ArrayList<Integer>());
        return result;
    }
    public void get(boolean[] item, int[] num, ArrayList<Integer> row){
        if(row.size() == len){
            result.add(new ArrayList<Integer>(row));
            return;
        }
        for(int i = 0; i < len; i ++){
            if(!item[i]){
                item[i] = true;
                row.add(num[i]);
                get(item, num, row);
                item[i] = false;
                row.remove(row.size() - 1);  
            }
        }
    }
}

 第三遍：

public class Solution {
    public List<List<Integer>> permute(int[] num) {
        ArrayList<List<Integer>> result = new ArrayList<List<Integer>>();
        getPermutation(num, num.length, new ArrayList<Integer>(), result);
        return result;
    }
    public void getPermutation(int[] num, int len, ArrayList<Integer> row, ArrayList<List<Integer>> result){
        if(num == null || len == 0){
            result.add(row);
            return;
        }
        for(int i = 0; i < len; i ++){
            ArrayList<Integer> nrow = new ArrayList<Integer>(row);
            nrow.add(num[i]);
            int tmp = num[i];
            num[i] = num[len - 1];
            getPermutation(num, len - 1, nrow, result);
            num[i] = tmp;
        }
    }
}