46. Permutations

swift/46-Permutations.swift

@@ -0,0 +1,42 @@
+class Solution {
+    var ans: [[Int]] = []
+
+    func permute(_ nums: [Int]) -> [[Int]] {
+        var referenceNums = nums
+        dfs(start: 0, currentOrder: &referenceNums)
+        return ans
+    }
+}
+
+private extension Solution {
+    private func swap(_ arr: inout [Int], at i1: Int, with i2: Int) {
+        guard 
+            i1 < arr.count,
+            i2 < arr.count
+        else { return }
+
+        let temp = 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
+        for i in 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)
+        }
+    }
+}