add: validate binary search tree

Author: JEONGBEOMKO

validate-binary-search-tree/JEONGBEOMKO.java

@@ -0,0 +1,34 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+/**
+ * time complexity: O(n)
+ * space complexity: O(h)
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return isValidRange(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    private boolean isValidRange(TreeNode root, long min, long max) {
+        if (root == null) return true;
+
+        if (root.val <= min || root.val >= max) return false;
+
+        return isValidRange(root.left, min, root.val)
+                && isValidRange(root.right, root.val, max);
+    }
+
+}