Add kth-smallest-element-in-a-bst solution in TS

Author: Jeehay28

kth-smallest-element-in-a-bst/Jeehay28.ts

@@ -0,0 +1,27 @@
+class TreeNode {
+  val: number;
+  left: TreeNode | null;
+  right: TreeNode | null;
+  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.left = left === undefined ? null : left;
+    this.right = right === undefined ? null : right;
+  }
+}
+
+// TC: O(n)
+// SC: O(n)
+function kthSmallest(root: TreeNode | null, k: number): number {
+  const result: number[] = [];
+  const dfs = (node: TreeNode | null) => {
+    if (!node) return;
+
+    dfs(node.left);
+    result.push(node.val);
+    dfs(node.right);
+  };
+
+  dfs(root);
+
+  return result[k - 1];
+}