diff --git a/solutions/compound-types/string.md b/solutions/compound-types/string.md
index e69de29..db02eab 100644
--- a/solutions/compound-types/string.md
+++ b/solutions/compound-types/string.md
@@ -0,0 +1,174 @@
+1. 
+```rust
+fn main() {
+    let s: &str = "hello, world";
+ }
+```
+
+2. 
+```rust
+fn main() {
+    let s: Box<str> =  "hello, world".into();
+    greetings(&s)
+ }
+ 
+ fn greetings(s: &str) {
+     println!("{}",s)
+ }
+```
+
+3. 
+```rust
+fn main() {
+    let mut s = String::new();
+    s.push_str("hello, world");
+    s.push('!');
+ 
+    assert_eq!(s, "hello, world!");
+ }
+```
+
+4. 
+```rust
+fn main() {
+    let mut s =  String::from("hello");
+     s.push(',');
+     s.push_str(" world");
+     s += "!";
+ 
+     println!("{}", s)
+ }
+```
+
+5. 
+```rust
+fn main() {
+    let s = String::from("I like dogs");
+    // Allocate new memory and store the modified string there
+    let s1 = s.replace("dogs", "cats");
+ 
+    assert_eq!(s1, "I like cats")
+ }
+```
+
+6. 
+```rust
+fn main() {
+    let s1 = String::from("hello,");
+    let s2 = String::from("world!");
+    let s3 = s1.clone() + &s2; 
+    assert_eq!(s3,"hello,world!");
+    println!("{}",s1);
+}
+```
+
+7. 
+```rust
+fn main() {
+    let s =  "hello, world".to_string();
+    greetings(s)
+}
+
+fn greetings(s: String) {
+    println!("{}",s)
+}
+```
+
+```rust
+fn main() {
+    let s = String::from("hello, world");
+    greetings(s)
+}
+
+fn greetings(s: String) {
+    println!("{}",s)
+}
+```
+
+8. 
+```rust
+fn main() {
+    let s =  "hello, world".to_string();
+    let s1: &str = &s;
+}
+```
+
+```rust
+fn main() {
+    let s =  "hello, world";
+    let s1: &str = s;
+}
+```
+
+```rust
+fn main() {
+    let s =  "hello, world".to_string();
+    let s1: String = s;
+}
+```
+
+9. 
+```rust
+fn main() {
+    // You can use escapes to write bytes by their hexadecimal values
+    // fill the blank below to show "I'm writing Rust"
+    let byte_escape = "I'm writing Ru\x73\x74!";
+    println!("What are you doing\x3F (\\x3F means ?) {}", byte_escape);
+
+    // ...or Unicode code points.
+    let unicode_codepoint = "\u{211D}";
+    let character_name = "\"DOUBLE-STRUCK CAPITAL R\"";
+
+    println!("Unicode character {} (U+211D) is called {}",
+                unicode_codepoint, character_name );
+
+   let long_string = "String literals
+                        can span multiple lines.
+                        The linebreak and indentation here \
+                         can be escaped too!";
+    println!("{}", long_string);
+}
+```
+
+10. 
+```rust
+fn main() {
+    let raw_str = "Escapes don't work here: \x3F \u{211D}";
+    // modify below line to make it work
+    assert_eq!(raw_str, "Escapes don't work here: ? ℝ");
+
+    // If you need quotes in a raw string, add a pair of #s
+    let quotes = r#"And then I said: "There is no escape!""#;
+    println!("{}", quotes);
+
+    // If you need "# in your string, just use more #s in the delimiter.
+    // You can use up to 65535 #s.
+    let  delimiter = r###"A string with "# in it. And even "##!"###;
+    println!("{}", delimiter);
+
+    // fill the blank
+    let long_delimiter = r###"Hello, "##""###;
+    assert_eq!(long_delimiter, "Hello, \"##\"")
+}
+```
+
+11. 
+```rust
+fn main() {
+    let s1 = String::from("hi,中国");
+    let h = &s1[0..1]; 
+    assert_eq!(h, "h");
+
+    let h1 = &s1[3..6];
+    assert_eq!(h1, "中");
+}
+```
+
+12. 
+```rust
+fn main() {
+    for c in "你好，世界".chars() {
+        println!("{}", c)
+    }
+}
+```
\ No newline at end of file
diff --git a/solutions/ownership/borrowing.md b/solutions/ownership/borrowing.md
index e69de29..04301ed 100644
--- a/solutions/ownership/borrowing.md
+++ b/solutions/ownership/borrowing.md
@@ -0,0 +1,143 @@
+1. 
+```rust
+fn main() {
+    let x = 5;
+    // fill the blank
+    let p = &x;
+ 
+    println!("the memory address of x is {:p}", p); // one possible output: 0x16fa3ac84
+}
+```
+
+2. 
+```rust
+fn main() {
+    let x = 5;
+    let y = &x;
+
+    // modify this line only
+    assert_eq!(5, *y);
+}
+```
+
+3. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    borrow_object(&s)
+}
+
+fn borrow_object(s: &String) {}
+```
+
+4. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    push_str(&mut s)
+}
+
+fn push_str(s: &mut String) {
+    s.push_str("world")
+}
+```
+
+5. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    // fill the blank to make it work
+    let p = &mut s;
+    
+    p.push_str("world");
+}
+```
+
+6. 
+```rust
+fn main() {
+    let c = '中';
+
+    let r1 = &c;
+    // fill the blank，dont change other code
+    let ref r2 = c;
+
+    assert_eq!(*r1, *r2);
+    
+    // check the equality of the two address strings
+    assert_eq!(get_addr(r1),get_addr(r2));
+}
+
+// get memory address string
+fn get_addr(r: &char) -> String {
+    format!("{:p}", r)
+}
+```
+
+7. 
+```rust
+fn main() {
+    let s = String::from("hello");
+
+    let r1 = &s;
+    let r2 = &s;
+
+    println!("{}, {}", r1, r2);
+}
+```
+
+8. 
+```rust
+fn main() {
+    //fix error by modifying this line
+    let mut s = String::from("hello, ");
+
+    borrow_object(&mut s)
+}
+
+fn borrow_object(s: &mut String) {}
+```
+
+9. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    borrow_object(&s);
+    
+    s.push_str("world");
+}
+
+fn borrow_object(s: &String) {}
+```
+
+10. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    let r1 = &mut s;
+    r1.push_str("world");
+    let r2 = &mut s;
+    r2.push_str("!");
+    
+    // println!("{}",r1);
+}
+```
+
+11. 
+```rust
+fn main() {
+    let mut s = String::from("hello, ");
+
+    let r1 = &mut s;
+    let r2 = &mut s;
+
+    // add one line below to make a compiler error: cannot borrow `s` as mutable more than once at a time
+    // you can't use r1 and r2 at the same time
+    r1.push_str("world");
+}
+```
\ No newline at end of file
diff --git a/src/compound-types/string.md b/src/compound-types/string.md
index 152165b..fdb29bb 100644
--- a/src/compound-types/string.md
+++ b/src/compound-types/string.md
@@ -3,7 +3,7 @@ The type of string literal `"hello, world"` is `&str`, e.g `let s: &str = "hello
 
 
 ### str and &str
-🌟 We can't use `str` type in normal ways, but we can use `&str`
+1. 🌟 We can't use `str` type in normal ways, but we can use `&str`
 
 ```rust,editable
 
@@ -14,7 +14,7 @@ fn main() {
 ```
 
 
-🌟🌟🌟 We can only use `str` by boxed it, `&` can be used to convert `Box<str>` to `&str` 
+2. 🌟🌟 We can only use `str` by boxed it, `&` can be used to convert `Box<str>` to `&str` 
 
 ```rust,editable
 
@@ -32,7 +32,7 @@ fn greetings(s: &str) {
 ### String
 `String` type is defined in std and stored as a vector of bytes (Vec<u8>), but guaranteed to always be a valid UTF-8 sequence. String is heap allocated, growable and not null terminated.
 
-🌟
+3. 🌟
 ```rust,editable
 
 // fill the blank
@@ -45,13 +45,13 @@ fn main() {
 }
 ```
 
-🌟🌟🌟
+4. 🌟🌟🌟
 ```rust,editable
 
 // fix all errors without adding newline
 fn main() {
    let  s =  String::from("hello");
-    s.push(',');`
+    s.push(',');
     s.push(" world");
     s += "!".to_string();
 
@@ -59,7 +59,7 @@ fn main() {
 }
 ```
 
-🌟🌟 `replace` can be used to replace substring
+5. 🌟🌟 `replace` can be used to replace substring
 ```rust,editable
 
 // fill the blank
@@ -74,11 +74,11 @@ fn main() {
 
 More `String` methods can be found under [String](https://doc.rust-lang.org/std/string/struct.String.html) module.
 
-🌟🌟 You can only concat a `String` with `&str`, and `String`'s ownership can be moved to another variable
+6. 🌟🌟 You can only concat a `String` with `&str`, and `String`'s ownership can be moved to another variable
 
 ```rust,editable
 
-// fix errors
+// fix errors without removing any line
 fn main() {
     let s1 = String::from("hello,");
     let s2 = String::from("world!");
@@ -91,7 +91,7 @@ fn main() {
 ### &str and String
 Opsite to the seldom using of `str`, `&str` and `String` are used everywhere!
 
-🌟🌟 `&str` can be converted to `String` in two ways
+7. 🌟🌟 `&str` can be converted to `String` in two ways
 ```rust,editable
 
 // fix error with at lest two solutions
@@ -105,7 +105,7 @@ fn greetings(s: String) {
 }
 ```
  
-🌟🌟 We can use `String::from` or `to_string` to convert a `&str` to `String`
+8. 🌟🌟 We can use `String::from` or `to_string` to convert a `&str` to `String`
 
 ```rust,editable
 
@@ -117,7 +117,7 @@ fn main() {
 ```
 
 ### string escapes
-🌟 
+9. 🌟 
 ```rust,editable
 fn main() {
     // You can use escapes to write bytes by their hexadecimal values
@@ -140,7 +140,7 @@ fn main() {
 }
 ```
 
-🌟🌟🌟 Sometimes there are just too many characters that need to be escaped or it's just much more convenient to write a string out as-is. This is where raw string literals come into play.
+10. 🌟🌟🌟 Sometimes there are just too many characters that need to be escaped or it's just much more convenient to write a string out as-is. This is where raw string literals come into play.
 
 ```rust,editable
 
@@ -211,7 +211,7 @@ fn main() {
 A more detailed listing of the ways to write string literals and escape characters is given in the ['Tokens' chapter](https://doc.rust-lang.org/reference/tokens.html) of the Rust Reference.
 
 ### string index
-🌟🌟 You can't use index to access a char in a string, but you can use slice `&s1[start..end]`.
+11. 🌟🌟🌟 You can't use index to access a char in a string, but you can use slice `&s1[start..end]`.
 
 ```rust,editable
 
@@ -226,7 +226,7 @@ fn main() {
 ```
 
 ### operate on UTF8 string
-🌟
+12. 🌟
 ```rust,editable
 
 fn main() {
diff --git a/src/ownership/borrowing.md b/src/ownership/borrowing.md
index 23d1354..1d5b87c 100644
--- a/src/ownership/borrowing.md
+++ b/src/ownership/borrowing.md
@@ -1,7 +1,7 @@
 # Reference and Borrowing
 
 ### Reference
-🌟
+1. 🌟
 ```rust,editable
 
 fn main() {
@@ -9,11 +9,11 @@ fn main() {
    // fill the blank
    let p = __;
 
-   println!("the memory address of x is {:p}", p); // output: 0x16fa3ac84
+   println!("the memory address of x is {:p}", p); // one possible output: 0x16fa3ac84
 }
 ```
 
-🌟
+2. 🌟
 ```rust,editable
 
 fn main() {
@@ -25,7 +25,7 @@ fn main() {
 }
 ```
 
-🌟
+3. 🌟
 ```rust,editable
 
 // fix error
@@ -38,20 +38,22 @@ fn main() {
 fn borrow_object(s: &String) {}
 ```
 
-🌟
+4. 🌟
 ```rust,editable
 
 // fix error
 fn main() {
     let mut s = String::from("hello, ");
 
-    borrow_object(&s)
+    push_str(s)
 }
 
-fn borrow_object(s: &mut String) {}
+fn push_str(s: &mut String) {
+    s.push_str("world")
+}
 ```
 
-🌟🌟
+5. 🌟🌟
 ```rust,editable
 
 fn main() {
@@ -67,7 +69,7 @@ fn main() {
 #### ref
 `ref` can be used to take references to a value, similar to `&`.
 
-🌟🌟🌟
+6. 🌟🌟🌟
 ```rust,editable
 
 fn main() {
@@ -90,7 +92,7 @@ fn get_addr(r: &char) -> String {
 ```
 
 ### Borrowing rules
-🌟
+7. 🌟
 ```rust,editable
 
 // remove something to make it work
@@ -106,7 +108,7 @@ fn main() {
 ```
 
 #### Mutablity
-🌟 Error: Borrow a immutable object as mutable
+8. 🌟 Error: Borrow a immutable object as mutable
 ```rust,editable
 
 fn main() {
@@ -119,7 +121,7 @@ fn main() {
 fn borrow_object(s: &mut String) {}
 ```
 
-🌟🌟 Ok: Borrow a mutable object as immutable
+9. 🌟🌟 Ok: Borrow a mutable object as immutable
 ```rust,editable
 
 // this code has no errors!
@@ -135,7 +137,7 @@ fn borrow_object(s: &String) {}
 ```
 
 ### NLL
-🌟🌟
+10. 🌟🌟
 ```rust,editable
 
 // comment one line to make it work
@@ -151,7 +153,7 @@ fn main() {
 }
 ```
 
-🌟🌟🌟
+11. 🌟🌟
 ```rust,editable
 
 fn main() {
diff --git a/zh-CN/src/compound-types/string.md b/zh-CN/src/compound-types/string.md
index 1baf5c4..9766653 100644
--- a/zh-CN/src/compound-types/string.md
+++ b/zh-CN/src/compound-types/string.md
@@ -14,7 +14,7 @@ fn main() {
 ```
 
 
-🌟🌟🌟 如果要使用 `str` 类型，只能配合 `Box`。  `&` 可以用来将 `Box<str>` 转换为 `&str` 类型
+🌟🌟 如果要使用 `str` 类型，只能配合 `Box`。  `&` 可以用来将 `Box<str>` 转换为 `&str` 类型
 
 ```rust,editable
 
@@ -51,7 +51,7 @@ fn main() {
 // 修复所有错误，并且不要新增代码行
 fn main() {
    let  s =  String::from("hello");
-    s.push(',');`
+    s.push(',');
     s.push(" world");
     s += "!".to_string();
 
@@ -79,7 +79,7 @@ fn main() {
 
 ```rust,editable
 
-// 修复所有错误
+// 修复所有错误，不要删除任何一行代码
 fn main() {
     let s1 = String::from("hello,");
     let s2 = String::from("world!");
diff --git a/zh-CN/src/ownership/borrowing.md b/zh-CN/src/ownership/borrowing.md
index f29fabb..d1caac2 100644
--- a/zh-CN/src/ownership/borrowing.md
+++ b/zh-CN/src/ownership/borrowing.md
@@ -45,10 +45,12 @@ fn borrow_object(s: &String) {}
 fn main() {
     let mut s = String::from("hello, ");
 
-    borrow_object(&s)
+    push_str(s)
 }
 
-fn borrow_object(s: &mut String) {}
+fn push_str(s: &mut String) {
+    s.push_str("world")
+}
 ```
 
 🌟🌟
@@ -151,7 +153,7 @@ fn main() {
 }
 ```
 
-🌟🌟🌟
+🌟🌟
 ```rust,editable
 
 fn main() {