【DFS】47.全排列II

题目

法1：DFS

最佳解法，类似阶乘公式！！！

class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> numsList = new ArrayList<>();
        for (int val : nums) {
            numsList.add(val);
        }
        dfs(numsList, 0, res);

        return res;
    }

    public void dfs(List<Integer> numsList, int idx, List<List<Integer>> res) {
        if (idx == numsList.size() - 1) {
            res.add(new ArrayList<>(numsList));
            return;
        }
        HashSet<Integer> set = new HashSet<>(); // 不加set则是普通全排列的解法
        for (int i = idx; i < numsList.size(); ++i) {
            if (set.contains(numsList.get(i))) {
                continue;
            }
            set.add(numsList.get(i));
            swap (numsList, idx, i);
            dfs(numsList, idx + 1, res);
            swap (numsList, idx, i);
        }
    }

    public void swap (List<Integer> numsList, int i, int j) {
        int tmp = numsList.get(i);
        numsList.set(i, numsList.get(j));
        numsList.set(j, tmp);
    }
}