力扣算法 Java 刷题笔记【回溯算法篇 DFS】hot100（一）全排列 、子集 、组合 3

文章目录

• 1. 全排列（中等）
• 2. 子集（中等）
• 3. 组合（中等）

1. 全排列（中等）

地址: https://leetcode-cn.com/problems/permutations/
2022/01/23
做题反思：

class Solution {
    List<List<Integer>> res = new LinkedList<>();
    public List<List<Integer>> permute(int[] nums) {
        LinkedList<Integer> track = new LinkedList<>();
        backtrack(nums, track);
        return res;
    }
    void backtrack(int[] nums, LinkedList<Integer> track) {
        if (track.size() == nums.length) {
            res.add(new LinkedList(track));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (track.contains(nums[i])) {
                continue;
            }
            track.add(nums[i]);
            backtrack(nums, track);
            track.removeLast();
        }
    }
}

2. 子集（中等）

地址: https://leetcode-cn.com/problems/subsets/
2022/01/23
做题反思：

class Solution {
    List<List<Integer>> res = new LinkedList<>();
    public List<List<Integer>> subsets(int[] nums) {
        LinkedList<Integer> track = new LinkedList<>();
        backtrack(nums, track, 0);
        return res;
    }
    void backtrack(int[] nums, LinkedList<Integer> track, int start) {
        res.add(new LinkedList(track));
        for (int i = start; i < nums.length; i++) {       
            track.add(nums[i]);
            backtrack(nums, track, i + 1);
            track.removeLast();
        }
    }
}

3. 组合（中等）

地址: https://leetcode-cn.com/problems/combinations/
2022/01/23
做题反思：

class Solution {
    List<List<Integer>> res = new LinkedList<>();
    public List<List<Integer>> combine(int n, int k) {
        LinkedList<Integer> track = new LinkedList<>();
        backtrack(n, k, track, 1);
        return res;
    }
    void backtrack(int n, int k, LinkedList<Integer> track, int start) {
        if (track.size() == k) {
            res.add(new LinkedList(track));
            return;
        }
        for (int i = start; i <= n; i++) {
            if (track.contains(i)) {
                continue;
            }
            track.add(i);
            backtrack(n, k, track, i + 1);
            track.removeLast();
        }
    }
}