46. 全排列

func permute(_ nums: [Int]) -> [[Int]] {
        let len = nums.count
        var exitArray = Array.init(repeating: 0, count: len)
        var tempArray = Array.init(repeating: 0, count: len)
        var ans = Array<Array<Int>>()
        dfs(0,nums,&tempArray, &exitArray,&ans)
        return ans
    }
    
    func dfs (_ index:Int,_ nums:[Int],_ tempArray:inout [Int],_ exitArray: inout [Int],_ ans: inout [[Int]]) {
        let len = nums.count
        if index == len {
            ans.append(tempArray)
            return
        }
        for i in 0..<len {
            if exitArray[i] == 0 {
                tempArray[index] = nums[i]
                exitArray[i] = 1
                dfs(index + 1,nums,&tempArray,&exitArray,&ans)
                //能走到这一行说明走到底了
                exitArray[i] = 0
            }
          
        }

    }

优化版本

func permute(_ nums: [Int]) -> [[Int]] {
        var tempNums = nums
        var ans = Array<Array<Int>>()
        dfs(0,&tempNums,&ans)
        return ans
    }

    func dfs (_ index:Int,_ nums:inout [Int],_ ans: inout [[Int]]) {
        let len = nums.count
        if index == len {
            ans.append(Array.init(nums))
            return
        }
        for i in index..<len {
            nums.swapAt(index, i)
            dfs(index + 1,&nums,&ans)
            nums.swapAt(index, i)
        }

    }