swift: add «216. Combination Sum III»

Signed-off-by: Matej Focko <me@mfocko.xyz>

swift/combination-sum-iii.swift

@@ -0,0 +1,40 @@
+class Solution {
+    private struct Solver {
+        let k, n: Int
+        var combinations: [[Int]]
+
+        init(k: Int, n: Int) {
+            self.k = k
+            self.n = n
+            self.combinations = []
+        }
+
+        mutating func solve() {
+            var combination: [Int] = []
+            solve(&combination, k: k, n: n, nextDigit: 1)
+        }
+
+        private mutating func solve(_ combination: inout [Int], k: Int, n: Int, nextDigit: Int) {
+            if k == 0 && n == 0 {
+                combinations.append(combination)
+                return
+            }
+
+            if k <= 0 || n <= 0 || nextDigit > 9 {
+                return
+            }
+
+            for d in nextDigit...9 {
+                combination.append(d)
+                solve(&combination, k: k - 1, n: n - d, nextDigit: d + 1)
+                _ = combination.popLast()
+            }
+        }
+    }
+
+    func combinationSum3(_ k: Int, _ n: Int) -> [[Int]] {
+        var s = Solver(k: k, n: n)
+        s.solve()
+        return s.combinations
+    }
+}