1 class Solution {
 2     func makesquare(_ nums: [Int]) -> Bool {
 3         if nums.isEmpty || nums.count < 4 {return false}
 4         var sum:Int = nums.reduce(0, +)
 5         if sum % 4 != 0 {return false}
 6         var n:Int = nums.count
 7         var all:Int = (1 << n) - 1
 8         var target:Int = sum / 4
 9         var masks:[Int] = [Int]()
10         var validHalf:[Bool] = [Bool](repeating:false,count:1 << n)
11         for i in 0...all
12         {
13             var curSum:Int = 0
14             for j in 0...15
15             {
16                 if ((i >> j) & 1) == 1
17                 {
18                     curSum += nums[j]
19                 }
20             }
21             if curSum == target
22             {
23                 for mask in masks
24                 {
25                     if (mask & i) != 0 {continue}
26                     var half:Int = mask | i
27                     validHalf[half] = true
28                     if validHalf[all ^ half] {return true}
29                 }
30                 masks.append(i)
31             }
32         }
33         return false
34     }
35 }

 1 class Solution {
 2     func makesquare(_ nums: [Int]) -> Bool {
 3         guard  nums.count >= 4 else { return false }
 4         let sum = nums.reduce(0, { $0 + $1 })
 5         if sum % 4 != 0 {
 6             return false
 7         }
 8         var sides: [Int] = Array(repeating: 0, count: 4)
 9         return dfs(nums.sorted(by: >), 0, &sides, sum / 4)
10     }
11
12     func dfs(_ nums: [Int], _ index: Int, _ sides: inout [Int], _ target: Int) -> Bool {
13         if index == nums.count {
14             return sides[0] == sides[1] && sides[0] == sides[2] && sides[0] == sides[3] && sides[0] == target
15         }
16
17         for i in 0 ..< sides.count {
18             if sides[i] + nums[index] > target {
19                 continue
20             }
21             sides[i] += nums[index]
22             if dfs(nums, index + 1, &sides, target) {
23                 return true
24             }
25             sides[i] -= nums[index]
26         }
27         return false
28     }
29 }