[Swift]LeetCode77. 組合 | Combinations

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公衆號：山青詠芝（shanqingyongzhi）
➤博客園地址：山青詠芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：http://www.javashuo.com/article/p-ftkwqqiy-me.html 
➤若是連接不是山青詠芝的博客園地址，則多是爬取做者的文章。
➤原文已修改更新！強烈建議點擊原文地址閱讀！支持做者！支持原創！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

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],
]

---

給定兩個整數 n 和 k，返回 1 ... n 中全部可能的 k 個數的組合。

示例:

輸入: n = 4, k = 2
輸出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

---

32ms

 1 class Solution {
 2     
 3     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 4         guard n > 0 else {
 5             return []
 6         }
 7         var result : [[Int]] = []
 8         var nums : [Int] = Array(repeating: 0, count: k)
 9         combineNext(lastIndex: 1,leftCount: k, numsIndex: 0, result: &result, nums: &nums, maxN: n)
10         return result
11     }
12     private func combineNext(lastIndex:Int,leftCount:Int,numsIndex:Int , result : inout  [[Int]] , nums : inout [Int] , maxN : Int) {
13         if leftCount == 0{
14             result.append(nums)
15             return
16         }
17         if lastIndex>maxN {
18             return
19         }
20             
21         for idx in lastIndex...(maxN-leftCount+1) {
22             nums[numsIndex] = idx
23             combineNext(lastIndex: idx+1, leftCount: leftCount-1,numsIndex: numsIndex+1, result: &result, nums: &nums, maxN: maxN)
24         }
25     }
26 }

---

36ms

 1 class Solution {
 2     
 3     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 4         guard n > 0 else {
 5             return []
 6         }
 7         var result = [[Int]]()
 8         helper(1, n, k, [Int](), &result)
 9         return result
10     }
11     
12     func helper(_ start: Int, _ end: Int, _ k: Int, _ added: [Int], _ result: inout [[Int]]) {
13         if k == 0 {
14             result.append(added)
15             return
16         }   
17         
18         for i in start...(end-k+1) {
19             var tempAdded = added
20             tempAdded.append(i)
21             helper(i+1, end, k-1, tempAdded, &result)
22         }
23     }
24 }

---

40ms

 1 class Solution {
 2     func combineHelper(withResult result: inout [[Int]], usingTemp temp: inout [Int], andStart start: Int, andN n: Int, andK k: Int) {
 3         
 4         if (k <= 0) {
 5             result.append(temp)
 6             return
 7         }
 8         
 9         for i in start...n {
10             if (n - i + 1) >= k {
11                 temp.append(i)
12                 combineHelper(withResult: &result, usingTemp: &temp, andStart: i + 1, andN: n, andK: k - 1)
13                 temp.removeLast()
14             }
15         }
16     }
17     
18     func combine(_ n: Int, _ k: Int) -> [[Int]] {
19         var result = [[Int]]();
20         var temp = [Int]()
21         combineHelper(withResult: &result, usingTemp: &temp, andStart: 1, andN: n, andK: k)
22         return result
23     }
24 }

---

48ms

 1 class Solution {
 2     
 3     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 4         guard n > 0 else {
 5             return []
 6         }
 7         var result = [[Int]]()
 8         helper(1, n, k, [Int](), &result)
 9         return result
10     }
11     
12     func helper(_ start: Int, _ end: Int, _ k: Int, _ added: [Int], _ result: inout [[Int]]) {
13         if k == 0 {
14             result.append(added)
15         }
16         
17         if start > end {
18             return
19         }      
20         
21         for i in start...(end-k+1) {
22             var tempAdded = added
23             tempAdded.append(i)
24             helper(i+1, end, k-1, tempAdded, &result)
25         }
26     }

---

68ms

 1 class Solution {
 2     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 3         if n < k {
 4             return []
 5         }
 6         var combo: [Int] = []
 7         var results: [[Int]] = []
 8         for i in 1...n {
 9             backtacking(i, n, k, &combo, &results)
10         }
11         return results
12     }
13     
14     private func backtacking(_ begin: Int, _ end: Int, _ limit: Int, _ combo: inout [Int], _ results: inout [[Int]]) {
15         let remains = end - begin + 1
16         if combo.count + remains >= limit {
17             combo.append(begin)
18             if combo.count == limit {
19                 results.append(combo)
20             } else {
21                 for i in 1..<remains {
22                     backtacking(begin + i, end, limit, &combo, &results)
23                 }
24             }
25             combo.removeLast()
26         }
27     }
28 }

---

116ms

 1 class Solution {
 2     
 3     func combine(_ n: Int, _ k: Int) -> [[Int]] {
 4         guard n > 0 else {
 5             return []
 6         }
 7         var result : [[Int]] = []
 8         var nums : [Int] = Array(repeating: 0, count: k)
 9         combineNext(lastIndex: 1,leftCount: k, numsIndex: 0, result: &result, nums: &nums, maxN: n)
10         return result
11     }
12     private func combineNext(lastIndex:Int,leftCount:Int,numsIndex:Int , result : inout  [[Int]] , nums : inout [Int] , maxN : Int) {
13         if leftCount == 0{
14             result.append(nums)
15             return
16         }
17         if lastIndex>maxN {
18             return
19         }
20             
21         for idx in lastIndex...maxN {
22             nums[numsIndex] = idx
23             combineNext(lastIndex: idx+1, leftCount: leftCount-1,numsIndex: numsIndex+1, result: &result, nums: &nums, maxN: maxN)
24         }
25     }
26 }

---

204ms

1 class Solution {
2     func combine(_ n: Int, _ k: Int) -> [[Int]] {
3         guard k <= n else { return [] }
4         guard k > 0 else { return [[]] }
5         guard k > 1 else { return (1...n).map { [$0] } }
6 
7         return combine(n-1, k) + combine(n-1, k-1).map { $0 + [n] }
8     }
9 }

---

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 }