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

1. 全排列（中等）2. 子集（中等）3. 组合（中等）

地址: https://leetcode-cn.com/problems/permutations/
2021/01/23
做题反思：

class Solution {
    List> res = new linkedList<>();
    public List> permute(int[] nums) {
        linkedList track = new linkedList<>();
        backtrack(nums, track);
        return res;
    }
    void backtrack(int[] nums, linkedList 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();
        }
    }
}

地址: https://leetcode-cn.com/problems/subsets/
2021/01/23
做题反思：

class Solution {
    List> res = new linkedList<>();
    public List> subsets(int[] nums) {
        linkedList track = new linkedList<>();
        backtrack(nums, track, 0);
        return res;
    }
    void backtrack(int[] nums, linkedList 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();
        }
    }
}

地址: https://leetcode-cn.com/problems/combinations/
2021/01/23
做题反思：

class Solution {
    List> res = new linkedList<>();
    public List> combine(int n, int k) {
        linkedList track = new linkedList<>();
        backtrack(n, k, track, 1);
        return res;
    }
    void backtrack(int n, int k, linkedList 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();
        }
    }
}