0046-permutations.swift

classSolution{
varans:[[Int]]=[]

func permute(_ nums:[Int])->[[Int]]{
varreferenceNums= nums
dfs(start:0, currentOrder:&referenceNums)
return ans
}
}

privateextensionSolution{
privatefunc swap(_ arr:inout[Int], at i1:Int, with i2:Int){
guard
 i1 < arr.count,
 i2 < arr.count
else{return}

lettemp=arr[i1]
arr[i1]=arr[i2]
arr[i2]= temp
}

func dfs(start:Int, currentOrder nums:inout[Int]){
 // base case [take into final answer]
if start == nums.count {
 ans.append(nums)
return
}

 // for each idx, take, switch with start, backtrack, switch back
foriin start..<nums.count {
 // choose i.e. take it
swap(&nums, at: i, with: start)

 // backtrack (from next idx)
dfs(start: start +1, currentOrder:&nums)

 // unchoose i.e. remove it
swap(&nums, at: start, with: i)
}
}
}