[Swift]LeetCode78. 子集 | Subsets

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：https://www.cnblogs.com/strengthen/p/9933967.html 
➤如果链接不是山青咏芝的博客园地址，则可能是爬取作者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持作者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

热烈欢迎，请直接点击！！！

进入博主App Store主页，下载使用各个作品！！！

注：博主将坚持每月上线一个新app！！！

Given a set of distinct integers, nums, return all possible subsets (the power set).

Note: The solution set must not contain duplicate subsets.

Example:

Input: nums = [1,2,3]
Output:
[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

---

给定一组不含重复元素的整数数组 nums，返回该数组所有可能的子集（幂集）。

说明：解集不能包含重复的子集。

示例:

输入: nums = [1,2,3]
输出:
[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

---

8ms

class Solution {
    func subsets(_ nums: [Int]) -> [[Int]] {
        var res = [[Int]()]
        subset(&res, nums)
        return res
    }

    private func subset(_ res: inout [[Int]], _ nums: [Int]) {
        guard let n = nums.first else { return }
        res += res.map { $0 + [n] }
        subset(&res, Array(nums[1..<nums.count]))
    }
}

---

12ms

class Solution {
    func subsets(_ nums: [Int]) -> [[Int]] {
        if nums.count == 0 {
            return []
        }
        
        var result = [[Int]]()
        var subset = [Int]()
        //call backtracking
        backtracking(nums, &result, &subset, 0)
        return result
    }
    
    func backtracking(_ nums: [Int], _ result: inout [[Int]], _ subset: inout [Int], _ startIndex: Int) {
        result.append(subset)
        for i in startIndex..<nums.count {
            subset.append(nums[i])
            backtracking(nums, &result, &subset, i + 1)
            subset.removeLast()
        }
    }
}

---

12ms

class Solution {
    func subsets(_ nums: [Int]) -> [[Int]] {
        var res = [[Int]]()
        rec_subset([Int](), nums, &res)
        res.append([Int]())
        return res
    }
    
    func rec_subset(_ firstNums: [Int], _ restNum: [Int], _ res: inout [[Int]]) {
        if restNum.count == 0 {
            if firstNums.count != 0 {
                res.append(firstNums)
            }
            return
        }

        var tempRestNums = restNum;
        var tempFirstNums  = firstNums;
        let firstVal = tempRestNums.remove(at: 0)
        tempFirstNums.append(firstVal)
        rec_subset(tempFirstNums, tempRestNums, &res)

        var zeroFirstNums = firstNums;
        rec_subset(zeroFirstNums, tempRestNums, &res)
    }
} 

---

16ms

class Solution {
    var result = [[Int]]()
    func subsets(_ nums: [Int]) -> [[Int]] {
        let count = nums.count
        for i in 0...count {
            var changeArray = Array(repeatElement(0, count: i))
            combine(&changeArray, nums, 0, i, 0, count)
        }
        return result
    }
    
    func combine(_ combineArray: inout [Int], _ nums: [Int], _ i: Int, _ total: Int, _ j: Int, _ maxJ: Int) {
        if i == total {
            result.append(combineArray)
            return ;
        }
        
        for k in j..<maxJ {
            combineArray[i] = nums[k]
            combine(&combineArray, nums, i + 1, total, k + 1, maxJ)
        }
    }
}

---

16ms

class Solution {
    func subsets(_ nums: [Int]) -> [[Int]] {
        guard nums.count > 0 else {return []}
        var subsets = [[Int]]()
        subsets.append([])
        
        for num in nums {            
            subsets += subsets.map{$0 + [num]}
        }
        return subsets
    }
}

---

28ms

class Solution {
    func subsets(_ nums: [Int]) -> [[Int]] {
        if nums.isEmpty {
            return []
        }
        
        var res: [[Int]] = [[]]
        helper(nums, &res)
        return res
    }
    
    fileprivate func helper(_ nums: [Int], _ res: inout [[Int]]) {
        
        guard let first = nums.first else {
            return
        }
        
        res += res.map{$0 + [first]}
        helper(Array(nums[1..<nums.count]), &res)
    }
}