diff --git a/QUSTOJ/1851.cpp b/QUSTOJ/1851.cpp
new file mode 100644
index 0000000..37ad65d
--- /dev/null
+++ b/QUSTOJ/1851.cpp
@@ -0,0 +1,40 @@
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <algorithm>
+using namespace std;
+int a[2048];
+int b[2048];
+int x[4096];
+int c;
+int d;
+int main()
+{
+    int t;
+    scanf("%d",&t);
+    for(int i=0;i<t;i++)
+    {
+        scanf("%d",&a[c++]);
+    }
+    scanf("%d",&t);
+    for(int i=0;i<t;i++)
+    {
+        scanf("%d",&b[d++]);
+    }
+    int* p=set_difference(a,a+c,b,b+d,x);
+    int dd=p-x;
+    if(dd==0)
+    {
+        printf("NULL\n");
+        /// 题目似乎有问题...
+        /// printf("0\n");
+        return 0;
+    }
+    for(int i=0;i<dd-1;i++)
+    {
+        printf("%d ",x[i]);
+    }
+    printf("%d\n",x[dd-1]);
+    printf("%d\n",dd);
+    return 0;
+}