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]

.

原题链接：https://oj.leetcode.com/problems/permutations/

题目：给定一组数。返回其全排列。

思路：记录每个数是否被使用过，将未使用过的数增加到current中，current长度已满。则增加到result中。

public List<List<Integer>> permute(int[] num) {
if (num == null)
return null;
List<List<Integer>> result = new ArrayList<List<Integer>>();
if (num.length == 0)
return result;
permute(num, new boolean[num.length], new ArrayList<Integer>(), result);
return result;
}

public void permute(int[] num, boolean[] isused,
ArrayList<Integer> current, List<List<Integer>> result) {
if (current.size() == num.length) {
result.add(new ArrayList<Integer>(current));
return;
}
for (int i = 0; i < num.length; i++) {
if (!isused[i]) {
isused[i] = true;
current.add(num[i]);
permute(num, isused, current, result);
current.remove(current.size() - 1);
isused[i] = false;
}
}
}