diff --git a/README.md b/README.md
index 5524889..3c4f3bd 100644
--- a/README.md
+++ b/README.md
@@ -36,4 +36,4 @@ Chapter 10: 29/29
 
 Chapter 11: 15/15
 
-Chapter 12: 0/37
+Chapter 12: 1/37
diff --git a/ch12/UVa12538_rope.cpp b/ch12/UVa12538_rope.cpp
new file mode 100644
index 0000000..2144f0b
--- /dev/null
+++ b/ch12/UVa12538_rope.cpp
@@ -0,0 +1,45 @@
+// UVa12538 Version Controlled IDE
+// Rujia Liu
+// This code makes use of rope, a persistent string available in gcc's STL extensions.
+#include<cstdio>
+#include<iostream>
+#include<ext/rope>
+using namespace std;
+using namespace __gnu_cxx;
+
+const int maxn = 50000 + 5;
+const int maxlen = 100000 + 5;
+
+crope cur, versions[maxn];
+char s[maxlen];
+
+int main() {
+  int n;
+  scanf("%d", &n); // a single test case
+  int d = 0;
+  int vnow = 0;
+  while(n--) {
+    int op, p, c, v;
+    scanf("%d", &op);
+    if(op == 1) {
+      scanf("%d%s", &p, s);
+      p -= d;
+      cur.insert(p, s);
+      versions[++vnow] = cur;
+    }
+    else if(op == 2) {
+      scanf("%d%d", &p, &c);
+      p -= d; c -= d;
+      cur.erase(p-1, c);
+      versions[++vnow] = cur;
+    }
+    else {
+      scanf("%d%d%d", &v, &p, &c);
+      p -= d; v -= d; c -= d;
+      crope r = versions[v].substr(p-1, c);
+      d += count(r.begin(), r.end(), 'c');
+      cout << r << "\n";
+    }
+  }
+  return 0;
+}
diff --git a/ch12/readme.md b/ch12/readme.md
index 230d0a0..fdce117 100644
--- a/ch12/readme.md
+++ b/ch12/readme.md
@@ -20,7 +20,7 @@
 
 [暂缺]12-6 UVa1674 Lightning Energy Report
 
-[暂缺]12-7 UVa12538 Version Controlled IDE
+12-7 UVa12538 Version Controlled IDE
 
 [暂缺]12-8 UVa805 Polygon Intersections
 
diff --git a/ch7/UVa1601.cpp b/ch7/UVa1601.cpp
index e44b8ff..9bfe3e7 100644
--- a/ch7/UVa1601.cpp
+++ b/ch7/UVa1601.cpp
@@ -1,6 +1,7 @@
 // UVa1601 The Morning after Halloween
 // Rujia Liu
-
+// This code implements the simpliest yet efficient-enough algorithm I'm aware of
+// Readers are encouraged to experiment on other algorithms (especially for better efficiency!)
 #include<cstdio>
 #include<cstring>
 #include<cctype>
@@ -12,13 +13,6 @@ const int maxn = 150; // 75% cells plus 2 fake nodes
 const int dx[]={1,-1,0,0,0}; // 4 moves, plus "no move"
 const int dy[]={0,0,1,-1,0};
 
-/*
-char S[16][22];
-int id[16][16],pos[MAXN][2],nums;
-int adj[MAXN][5],deg[MAXN];
-int Q[MAXN*MAXN*MAXN],head,tail;
-*/
-
 inline int ID(int a, int b, int c) {
   return (a<<16)|(b<<8)|c;
 }