[Swift]LeetCode77. 组合 | Combinations

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➤微信公众号：山青咏芝（shanqingyongzhi）
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Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

---

给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。

示例:

输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

---

32ms

class Solution {
    
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        guard n > 0 else {
            return []
        }
        var result : [[Int]] = []
        var nums : [Int] = Array(repeating: 0, count: k)
        combineNext(lastIndex: 1,leftCount: k, numsIndex: 0, result: &result, nums: &nums, maxN: n)
        return result
    }
    private func combineNext(lastIndex:Int,leftCount:Int,numsIndex:Int , result : inout  [[Int]] , nums : inout [Int] , maxN : Int) {
        if leftCount == 0{
            result.append(nums)
            return
        }
        if lastIndex>maxN {
            return
        }
            
        for idx in lastIndex...(maxN-leftCount+1) {
            nums[numsIndex] = idx
            combineNext(lastIndex: idx+1, leftCount: leftCount-1,numsIndex: numsIndex+1, result: &result, nums: &nums, maxN: maxN)
        }
    }
}

---

36ms

class Solution {
    
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        guard n > 0 else {
            return []
        }
        var result = [[Int]]()
        helper(1, n, k, [Int](), &result)
        return result
    }
    
    func helper(_ start: Int, _ end: Int, _ k: Int, _ added: [Int], _ result: inout [[Int]]) {
        if k == 0 {
            result.append(added)
            return
        }   
        
        for i in start...(end-k+1) {
            var tempAdded = added
            tempAdded.append(i)
            helper(i+1, end, k-1, tempAdded, &result)
        }
    }
}

---

40ms

class Solution {
    func combineHelper(withResult result: inout [[Int]], usingTemp temp: inout [Int], andStart start: Int, andN n: Int, andK k: Int) {
        
        if (k <= 0) {
            result.append(temp)
            return
        }
        
        for i in start...n {
            if (n - i + 1) >= k {
                temp.append(i)
                combineHelper(withResult: &result, usingTemp: &temp, andStart: i + 1, andN: n, andK: k - 1)
                temp.removeLast()
            }
        }
    }
    
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        var result = [[Int]]();
        var temp = [Int]()
        combineHelper(withResult: &result, usingTemp: &temp, andStart: 1, andN: n, andK: k)
        return result
    }
}

---

48ms

class Solution {
    
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        guard n > 0 else {
            return []
        }
        var result = [[Int]]()
        helper(1, n, k, [Int](), &result)
        return result
    }
    
    func helper(_ start: Int, _ end: Int, _ k: Int, _ added: [Int], _ result: inout [[Int]]) {
        if k == 0 {
            result.append(added)
        }
        
        if start > end {
            return
        }      
        
        for i in start...(end-k+1) {
            var tempAdded = added
            tempAdded.append(i)
            helper(i+1, end, k-1, tempAdded, &result)
        }
    }

---

68ms

class Solution {
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        if n < k {
            return []
        }
        var combo: [Int] = []
        var results: [[Int]] = []
        for i in 1...n {
            backtacking(i, n, k, &combo, &results)
        }
        return results
    }
    
    private func backtacking(_ begin: Int, _ end: Int, _ limit: Int, _ combo: inout [Int], _ results: inout [[Int]]) {
        let remains = end - begin + 1
        if combo.count + remains >= limit {
            combo.append(begin)
            if combo.count == limit {
                results.append(combo)
            } else {
                for i in 1..<remains {
                    backtacking(begin + i, end, limit, &combo, &results)
                }
            }
            combo.removeLast()
        }
    }
}

---

116ms

class Solution {
    
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        guard n > 0 else {
            return []
        }
        var result : [[Int]] = []
        var nums : [Int] = Array(repeating: 0, count: k)
        combineNext(lastIndex: 1,leftCount: k, numsIndex: 0, result: &result, nums: &nums, maxN: n)
        return result
    }
    private func combineNext(lastIndex:Int,leftCount:Int,numsIndex:Int , result : inout  [[Int]] , nums : inout [Int] , maxN : Int) {
        if leftCount == 0{
            result.append(nums)
            return
        }
        if lastIndex>maxN {
            return
        }
            
        for idx in lastIndex...maxN {
            nums[numsIndex] = idx
            combineNext(lastIndex: idx+1, leftCount: leftCount-1,numsIndex: numsIndex+1, result: &result, nums: &nums, maxN: maxN)
        }
    }
}

---

204ms

class Solution {
    func combine(_ n: Int, _ k: Int) -> [[Int]] {
        guard k <= n else { return [] }
        guard k > 0 else { return [[]] }
        guard k > 1 else { return (1...n).map { [$0] } }

        return combine(n-1, k) + combine(n-1, k-1).map { $0 + [n] }
    }
}

---

152ms

 1 class Solution {
 2     var list:[[Int]] = [[Int]]()
 3     var n:Int = 0
 4     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 5         self.n = n
 6         var comb:[Int] = [Int]()
 7         combines(comb,1, k)
 8         return list
 9     }
10     
11     func combines(_ temp:[Int],_ count:Int,_ k:Int)
12     {
13         var temp = temp
14         if k == 0
15         {
16             list.append(temp)
17             return
18         }
19         if count + k - 1 > n { return}
20         temp.append(count)
21         combines(temp, count + 1, k - 1)
22         temp.removeLast()
23         combines(temp, count + 1, k)
24     }
25 }