add: leetcode 0700 solution

leetcode/0700.Search-in-a-Binary-Search-Tree/700.Search in a Binary Search Tree.go

@@ -0,0 +1,20 @@
+package leetcode
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func searchBST(root *TreeNode, val int) *TreeNode {
+	if root == nil {
+		return nil
+	}
+	if root.Val == val {
+		return root
+	} else if root.Val < val {
+		return searchBST(root.Right, val)
+	} else {
+		return searchBST(root.Left, val)
+	}
+}