添加 0078.子集.md Scala版本

problems/0078.子集.md

@@ -373,6 +373,60 @@ func subsets(_ nums: [Int]) -> [[Int]] {
 }
 ```
 
+## Scala
+
+思路一: 使用本题解思路
+
+```scala
+object Solution {
+  import scala.collection.mutable
+  def subsets(nums: Array[Int]): List[List[Int]] = {
+    var result = mutable.ListBuffer[List[Int]]()
+    var path = mutable.ListBuffer[Int]()
+
+    def backtracking(startIndex: Int): Unit = {
+      result.append(path.toList) // 存放结果
+      if (startIndex >= nums.size) {
+        return
+      }
+      for (i <- startIndex until nums.size) {
+        path.append(nums(i)) // 添加元素
+        backtracking(i + 1)
+        path.remove(path.size - 1) // 删除
+      }
+    }
+
+    backtracking(0)
+    result.toList
+  }
+}
+```
+
+思路二: 将原问题转换为二叉树，针对每一个元素都有**选或不选**两种选择，直到遍历到最后，所有的叶子节点即为本题的答案：
+
+```scala
+object Solution {
+  import scala.collection.mutable
+  def subsets(nums: Array[Int]): List[List[Int]] = {
+    var result = mutable.ListBuffer[List[Int]]()
+
+    def backtracking(path: mutable.ListBuffer[Int], startIndex: Int): Unit = {
+      if (startIndex == nums.length) {
+        result.append(path.toList)
+        return
+      }
+      path.append(nums(startIndex))
+      backtracking(path, startIndex + 1)  // 选择元素
+      path.remove(path.size - 1)
+      backtracking(path, startIndex + 1)  // 不选择元素
+    }
+
+    backtracking(mutable.ListBuffer[Int](), 0)
+    result.toList
+  }
+}
+```
+
 
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>