Add problem 98 .

1 files changed, 29 insertions, 0 deletions

diff --git a/cpp/98.cpp b/cpp/98.cpp
new file mode 100644
index 0000000..73cefa0
--- /dev/null
+++ b/cpp/98.cpp
@@ -0,0 +1,29 @@
+#include <cstddef>
+
+struct TreeNode
+{
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+#include <limits>
+
+class Solution
+{
+public:
+    bool dfs(TreeNode *node, long long min, long long max)
+    {
+        if (node == nullptr)
+            return true;
+        if (node->val <= min || node->val >= max)
+            return false;
+        return dfs(node->left, min, node->val) && dfs(node->right, node->val, max);
+    }
+
+    bool isValidBST(TreeNode *root)
+    {
+        return dfs(root, std::numeric_limits<long long>::min(), std::numeric_limits<long long>::max());
+    }
+};