chore: add hackerrank solutions

hacker-rank/tree-preorder-traversal/main.go

@@ -0,0 +1,52 @@
+package main
+import "fmt"
+import "os"
+import "bufio"
+import "strconv"
+import "strings"
+
+func main() {
+    var length int
+    var tree *node
+    
+    scanner := bufio.NewScanner(os.Stdin)
+    for scanner.Scan() {
+        if length == 0 {
+            length, _ = strconv.Atoi(scanner.Text())
+            continue
+        }
+   
+        for i, txt := range strings.Fields(scanner.Text()) {
+            if i >= length { break }
+            if v, err := strconv.Atoi(txt); err == nil {
+                tree = insert(tree, &node{value: v})
+            }
+        }
+    }
+    
+    fmt.Println(tree)
+}
+
+type node struct {
+    value int
+    left *node
+    right *node
+}
+
+func insert(root, n *node) *node {
+        if root == nil {
+            return n
+        }
+        if root.value > n.value {
+            root.left = insert(root.left, n)
+            return root
+        }
+        root.right = insert(root.right, n)
+        return root
+}
+
+func (n *node) String() string {
+    if n == nil { return "" }
+        return fmt.Sprintf("%v %v%v", n.value, n.left.String(), n.right.String())
+
+}