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  • [Swift]LeetCode18. 四数之和 | 4Sum

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    ➤微信公众号：山青咏芝（shanqingyongzhi）
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    ➤原文地址：https://www.cnblogs.com/strengthen/p/9889017.html 
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    Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

    Note:

    The solution set must not contain duplicate quadruplets.

    Example:

    Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]
    给定一个包含 n 个整数的数组 nums 和一个目标值 target，判断 nums 中是否存在四个元素 a，b，c 和 d ，使得 a + b + c + d 的值与 target 相等？找出所有满足条件且不重复的四元组。

    注意：

    答案中不可以包含重复的四元组。

    示例：

    给定数组 nums = [1, 0, -1, 0, -2, 2]，和 target = 0。

满足要求的四元组集合为：
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]
    40ms
     1 class Solution {
 2     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 3         let sortedNums = nums.sorted()
 4         var results = [[Int]]()
 5         findNSum(sortedNums, left: 0, right: sortedNums.count - 1, target:target, N: 4, result: [Int](), results: &results)
 6         return results
 7     }
 8     
 9     func findNSum(_ sortedNums: [Int], left: Int, right: Int, target: Int, N: Int, result: [Int], results: inout [[Int]]) {
10         if (right - left + 1) < N || N < 2 || target < N * sortedNums[left] || target > sortedNums[right] * N {
11             return
12         }
13         
14         if N > 2 {
15             for i in left...right {
16                 if i == left || sortedNums[i] != sortedNums[i-1] {
17                     var result = result
18                     result.append(sortedNums[i])
19                     findNSum(sortedNums, left: i+1, right: right, target: target-sortedNums[i], N: N-1, result: result, results: &results)
20                 }
21             }
22         } else {
23             var left = left
24             var right = right
25             while left < right {
26                 let sum = sortedNums[left] + sortedNums[right]
27                 if sum == target {
28                     results.append(result + [sortedNums[left], sortedNums[right]])
29                     left = left + 1
30                     while left < right && sortedNums[left] == sortedNums[left - 1] {
31                         left = left + 1
32                     }
33                 } else if sum > target {
34                     right = right - 1
35                 } else {
36                     left = left + 1
37                 }
38             }
39         }
40     }
41 }

    60ms

     1 class Solution {
 2     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 3         if nums.count < 4 {return []}
 4         let sorted = nums.sorted()     
 5         var quardruplets = [[Int]]()
 6         for i in 0..<(nums.count - 3) {
 7             if i > 0 && sorted[i] == sorted[i - 1] {continue}
 8             for j in (i + 1)..<(nums.count - 2) {
 9                 if j > i + 1 && sorted[j] == sorted[j - 1] {continue}
10                 var k = j + 1
11                 var l = nums.count - 1
12                 let twoSumTarget = target - sorted[i] - sorted[j]
13                 while k < l {
14                     let twoSum = sorted[k] + sorted[l]
15                     if twoSum < twoSumTarget {k += 1}
16                     if twoSum > twoSumTarget {l -= 1}
17                     if twoSum == twoSumTarget {
18                         quardruplets.append([sorted[i], sorted[j], sorted[k], sorted[l]]) 
19                         repeat {k += 1} while k < l && sorted[k] == sorted[k - 1]
20                         repeat {l -= 1} while k < l && sorted[l] == sorted[l + 1]
21                     }
22                 }  
23             }
24         }
25         return quardruplets
26     }
27 }

    72ms

     1 class Solution {
 2     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 3         let count = nums.count
 4         guard count >= 4 else { return [] }
 5         let nums = nums.sorted()
 6         
 7         var result = [[Int]]()
 8         for i in 0..<count - 3 {
 9             if i > 0 && nums[i] == nums[i - 1] {
10                 continue
11             }
12             var threeSum = target - nums[i]
13             
14             for j in i + 1..<count - 2 {
15                 if j > i + 1 && nums[j] == nums[j - 1] {
16                     continue
17                 }
18                 var twoSum = threeSum - nums[j]
19                 var left = j + 1, right = count - 1
20                 while left < right {
21                     if nums[left] + nums[right] == twoSum {
22                         result.append([nums[i], nums[j], nums[left], nums[right]])
23                         while left < right && nums[left] == nums[left + 1] {
24                             left += 1
25                         }
26                         while left < right && nums[right] == nums[right - 1] {
27                             right -= 1
28                         }
29                         left += 1
30                         right -= 1
31                     } else if nums[left] + nums[right] < twoSum {
32                         left += 1
33                     } else {
34                         right -= 1
35                     }
36                 }
37             }
38         }
39         return result
40     }
41 }

    88ms

     1 class Solution {
 2     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 3         var res: [[Int]]  =  [[Int]]()
 4         let len = nums.count
 5         if len < 4 {return res}
 6         var nums = nums.sorted(by: <)
 7         for i in 0..<len
 8         {
 9             if i > 0 && nums[i] == nums[i - 1]
10             {
11                 continue
12             }
13             for j in (i + 1)..<len
14             {
15                 //第二个数的去重处理
16                 if j > i + 1 && nums[j] == nums[j - 1]
17                 {
18                     continue
19                 }
20                 var l:Int = j + 1
21                 var r = len - 1
22                 while (l < r) 
23                 {
24                     var sum = nums[i] + nums[j] + nums[l] + nums[r]
25                     if sum == target
26                     {
27                         res.append([nums[i], nums[j], nums[l], nums[r]])
28                         while (l < r && nums[l] == nums[l + 1]) { l += 1}
29                         while (l < r && nums[r] == nums[r - 1]) { r -= 1}
30                         l += 1
31                         r -= 1
32                     }
33                     else if sum < target {l += 1}
34                     else {r -= 1}
35                 }
36             }
37         }
38         return res
39     }
40 }

    108ms

     1 class Solution {
 2     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 3         
 4         var array = nums.sorted()
 5         var result = [[Int]]()
 6         guard nums.count >= 4 else { return result }
 7         
 8         for i in 0 ..< nums.count - 3 {
 9             for j in i + 1 ..< nums.count - 2 {
10                 var left = j + 1
11                 var right = nums.count - 1
12                 while left < right {
13                     let sum = array[i] + array[j] + array[left] + array[right]
14                     if sum == target {
15                         let temp = [array[i], array[j], array[left], array[right]]
16                         if !result.contains(temp) {
17                             result.append(temp)   
18                         }
19                         left += 1
20                         right -= 1
21                     } else if sum > target {
22                         right -= 1
23                     } else {
24                         left += 1
25                     }
26                 }
27             }
28         }  
29         return result
30     }
31 }

    668ms

     1 class Solution {
 2     func twoSum(sortedNums: [Int], firstNumIndex: Int, secondNumIndex: Int, target: Int) -> [[Int]] {
 3         var low = secondNumIndex + 1
 4         var high = sortedNums.count - 1
 5         var result = [[Int]]()
 6         let firstNum = sortedNums[firstNumIndex]
 7         let secondNum = sortedNums[secondNumIndex]
 8 
 9         // 将 low 往右移，并跳过重复
10         func moveLow() {
11             repeat {
12                 low += 1
13             } while sortedNums[low] == sortedNums[low - 1] && low < high
14         }
15 
16         // 将 high 往左移，并跳过重复
17         func moveHigh() {
18             repeat {
19                 high -= 1
20             } while sortedNums[high] == sortedNums[high + 1] && low < high
21         }
22 
23         while low < high {
24             let sum = sortedNums[low] + sortedNums[high] + firstNum + secondNum
25 
26             if sum < target {
27                 moveLow()
28             } else if sum > target {
29                 moveHigh()
30             } else { // sum == target
31                 result.append([firstNum, secondNum, sortedNums[low], sortedNums[high]])
32 
33                 moveLow()
34                 moveHigh()
35             }
36         }
37 
38         return result
39     }
40 
41     func threeSum(sortedNums: [Int], firstNumIndex: Int, target: Int) -> [[Int]] {
42         var secondNumIndex = firstNumIndex + 1
43         var result = [[Int]]()
44 
45         while secondNumIndex < sortedNums.count - 2 {
46             result.append(contentsOf: twoSum(
47                 sortedNums: sortedNums,
48                 firstNumIndex: firstNumIndex,
49                 secondNumIndex: secondNumIndex,
50                 target: target
51             ))
52 
53             repeat {
54                 secondNumIndex += 1
55             } while sortedNums[secondNumIndex] == sortedNums[secondNumIndex - 1] &&
56                     secondNumIndex < sortedNums.count - 2
57         }
58 
59         return result
60     }
61 
62     func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
63         guard nums.count >= 4 else { return [] }
64         var result = [[Int]]()
65         var firstNumIndex = 0
66         let sortedNums = nums.sorted()
67 
68         while firstNumIndex < sortedNums.count - 3 {
69             result.append(contentsOf: threeSum(
70                 sortedNums: sortedNums,
71                 firstNumIndex: firstNumIndex,
72                 target: target
73             ))
74 
75             repeat {
76                 firstNumIndex += 1
77             } while sortedNums[firstNumIndex] == sortedNums[firstNumIndex - 1] &&
78                     firstNumIndex < sortedNums.count - 3
79         }
80 
81         return result
82     }
83 }
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  • 原文地址：https://www.cnblogs.com/strengthen/p/9889017.html
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