solve problem

Author: delight010

kth-smallest-element-in-a-bst/delight010.swift

@@ -0,0 +1,31 @@
+class Solution {
+    // Time O(k)
+    // Space O(height of Tree)
+    func kthSmallest(_ root: TreeNode?, _ k: Int) -> Int {
+        var count = 0
+        var result = root!.val
+        
+        inorderTree(root, &count, k, &result)
+        
+        return result
+    }
+    
+    private func inorderTree(_ node: TreeNode?, _ count: inout Int, _ k: Int, _ result: inout Int) {
+        guard let node = node else { return }
+        if count == k { return }
+        
+        // left search
+        inorderTree(node.left, &count, k, &result)
+        
+        // current node
+        count += 1
+        if count == k {
+            result = node.val
+            return
+        }
+        
+        // right search
+        inorderTree(node.right, &count, k, &result)
+    }
+}
+