LeetCode每日一题：permutations

问题描述

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].

问题分析

数组全排列问题，典型的递归算法，每次交换两位数字递归生成所有的数组就行了。

代码实现

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

    private void permuteSwap(int i, int[] num, ArrayList<ArrayList<Integer>> result) {
        ArrayList<Integer> list = new ArrayList<>();
        if (i == num.length) {
            for (int j = 0; j < num.length; j++) {
                list.add(num[j]);
            }
            result.add(list);
            return;
        }
        for (int j = i; j < num.length; j++) {
            int temp = num[i];
            num[i] = num[j];
            num[j] = temp;
            permuteSwap(i + 1, num, result);
            temp = num[i];
            num[i] = num[j];
            num[j] = temp;
        }
    }