[leetcode] Permutations

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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：產生permutation，這題假設沒有重複元素，遞迴求解，每一次在cur位置填元素，添之前判斷鈣元素是否已經用過，沒用過繼續下去，指導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 }));}}