[leetcode] 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].

https://oj.leetcode.com/problems/permutations/

思路：生成permutation，这题假设没有重复元素，递归求解，每一次在cur位置填元素，添之前判断钙元素是否已经用过，没用过继续下去，指导cur==n。

public class Solution {
public ArrayList<ArrayList<Integer>> permute(int[] num) {
if (num == null)
return null;
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
if (num.length == 0)
return res;
int[] permSeq = new int[num.length];
perm(num.length, 0, num, permSeq, res);
return res;
}

private void perm(int n, int cur, int[] num, int[] perm,
ArrayList<ArrayList<Integer>> res) {
if (cur == n) {
ArrayList<Integer> tmp = new ArrayList<Integer>();
for (int i = 0; i < perm.length; i++) {
tmp.add(perm[i]);
}
res.add(tmp);
} else {
int i;
for (i = 0; i < num.length; i++) {
int j;
boolean ok = true;
for (j = cur - 1; j >= 0; j--) {
if (perm[j] == num[i])
ok = false;
}
if (ok) {
perm[cur] = num[i];
perm(n, cur + 1, num, perm, res);
}
}
}

}

public static void main(String[] args) {
System.out.println(new Solution().permute(new int[] { 1, 2, 3,4 }));
}
}