add solution for validate binary search tree

Author: youngDaLee

validate-binary-search-tree/youngDaLee.go

@@ -0,0 +1,24 @@
+package youngDaLee
+
+import "math"
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func isValidBST(root *TreeNode) bool {
+	return dfs(root, math.MinInt, math.MaxInt)
+}
+
+func dfs(node *TreeNode, min, max int) bool {
+	if node == nil {
+		return true
+	}
+	if (min < node.Val && node.Val < max) == false {
+		return false
+	}
+
+	return dfs(node.Left, min, node.Val) && dfs(node.Right, node.Val, max)
+}