diff --git a/098_validate_binary_search_tree/Makefile b/098_validate_binary_search_tree/Makefile
new file mode 100644
index 0000000..72cc896
--- /dev/null
+++ b/098_validate_binary_search_tree/Makefile
@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test valid_bst.c
diff --git a/098_validate_binary_search_tree/valid_bst.c b/098_validate_binary_search_tree/valid_bst.c
new file mode 100644
index 0000000..f112d5b
--- /dev/null
+++ b/098_validate_binary_search_tree/valid_bst.c
@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+struct TreeNode {
+    int val;
+    struct TreeNode *left;
+    struct TreeNode *right;
+};
+
+static int last_val = -1;
+
+static bool traverse(struct TreeNode* node)
+{
+    bool ret = true;
+
+    if (ret && node->left != NULL) {
+        ret = traverse(node->left);
+    }
+
+    if (last_val != -1 && node->val <= last_val) {
+        return false;
+    }
+    last_val = node->val;
+
+    if (ret && node->right != NULL) {
+        ret = traverse(node->right);
+    }
+    return ret;
+}
+
+static bool isValidBST(struct TreeNode* root)
+{
+    if (root == NULL)  return true;
+    last_val = -1;
+    return traverse(root);
+}
+
+int main(int argc, char **argv)
+{
+    struct TreeNode root;
+    struct TreeNode left;
+    struct TreeNode right;
+    root.val = 2;
+    root.left = &left;
+    root.right = &right;
+    left.val = 1;
+    left.left = NULL;
+    left.right = NULL;
+    right.val = 3;
+    right.left = NULL;
+    right.right = NULL;
+    printf("%s\n", isValidBST(&root) ? "true" : "false");
+    return 0;
+}
diff --git a/099_recover_binary_search_tree/Makefile b/099_recover_binary_search_tree/Makefile
new file mode 100644
index 0000000..95d5434
--- /dev/null
+++ b/099_recover_binary_search_tree/Makefile
@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test recover_bst.c
diff --git a/099_recover_binary_search_tree/recover_bst.c b/099_recover_binary_search_tree/recover_bst.c
new file mode 100644
index 0000000..ecae963
--- /dev/null
+++ b/099_recover_binary_search_tree/recover_bst.c
@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+struct TreeNode {
+    int val;
+    struct TreeNode *left;
+    struct TreeNode *right;
+};
+
+static struct TreeNode *last = NULL;
+static struct TreeNode *m1 = NULL;
+static struct TreeNode *m2 = NULL;
+static int wrong = 0;
+
+static void traverse(struct TreeNode* node)
+{
+    if (node->left != NULL) {
+        traverse(node->left);
+    }
+
+    if (last != NULL && node->val < last->val) {
+        if (++wrong == 2) {
+            int tmp = node->val;
+            node->val = m1->val;
+            m1->val = tmp;
+        }
+        m1 = last;
+        m2 = node;
+    }
+    last = node;
+
+    if (node->right != NULL) {
+        traverse(node->right);
+    }
+}
+
+static void recoverTree(struct TreeNode* root) {
+    if (root != NULL) {
+        last = NULL;
+        m1 = NULL;
+        m2 = NULL;
+        wrong = 0;
+        traverse(root);
+        if (wrong == 1) {
+            int tmp = m1->val;
+            m1->val = m2->val;
+            m2->val = tmp;
+        }        
+    }
+}
+
+int main(int argc, char **argv)
+{
+    struct TreeNode root;
+    struct TreeNode left;
+    struct TreeNode right;
+    root.val = 2;
+    root.left = &left;
+    root.right = &right;
+    left.val = 3;
+    left.left = NULL;
+    left.right = NULL;
+    right.val = 1;
+    right.left = NULL;
+    right.right = NULL;
+    recoverTree(&root);
+    printf("%d %d %d\n", root.val, left.val, right.val);
+    return 0;
+}