19 August 2020
LeetCode(46) -- Permutations
by Jerry Zhang
Problem
Given a collection of distinct integers, return all possible permutations.
Example:
Input: [1,2,3]
Output:
[
[1,2,3],
[1,3,2],
[2,1,3],
[2,3,1],
[3,1,2],
[3,2,1]
]
Solution
以前做过这道题,今天试着自己写,还是有bug。下面重新整理一下答案的思路吧。
backtrack这个方法中,把所有元素都转化成list,参数中还包括元素的个数,以及开始的位置。然后分别对调每个字母都后面所有字母的位置,递归使用backtrack,最后再对调回来。
public class Permutation {
public List<List<Integer>> permute(int[] nums) {
List<List<Integer>> output = new LinkedList<>();
ArrayList<Integer> list = new ArrayList<>();
for (int num : nums) {
list.add(num);
}
int n = nums.length;
backtrack(n, list, output, 0);
return output;
}
public void backtrack(int n, ArrayList<Integer> nums, List<List<Integer>> output, int first) {
if (first == n) {
output.add(new ArrayList<>(nums));
}
for (int i = first; i < n; i++) {
Collections.swap(nums, first, i);
backtrack(n, nums, output, first + 1);
Collections.swap(nums, first, i);
}
}
public static void main(String[] args) {
System.out.println(new Permutation().permute(new int[]{1, 2, 3}));
}
}