Add English translations for String data structure

Author: marvinyan

en/basics_data_structure/string.md

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+# String
+
+String-related problems often appear in interview questions. In actual
+development, strings are also frequently used. Summarized here are common uses
+of strings in C++, Java, and Python.
+
+## Python
+
+```python
+s1 = str()
+# in python, `''` and `""` are the same
+s2 = "shaunwei" # 'shaunwei'
+s2len = len(s2)
+# last 3 chars
+s2[-3:] # wei
+s2[5:8] # wei
+s3 = s2[:5] # shaun
+s3 += 'wei' # return 'shaunwei'
+# list in python is same as ArrayList in java
+s2list = list(s3)
+# string at index 4
+s2[4] # 'n'
+# find index at first
+s2.index('w')  # return 5, if not found, throw ValueError
+s2.find('w') # return 5, if not found, return -1
+```
+
+In Python, there's no StringBuffer or StringBuilder. However, string manipulations
+are fairly efficient already.
+
+## Java
+
+```java
+String s1 = new String();
+String s2 = "billryan";
+int s2Len = s2.length();
+s2.substring(4, 8); // return "ryan"
+StringBuilder s3 = new StringBuilder(s2.substring(4, 8));
+s3.append("bill");
+String s2New = s3.toString(); // return "ryanbill"
+// convert String to char array
+char[] s2Char = s2.toCharArray();
+// char at index 4
+char ch = s2.charAt(4); // return 'r'
+// find index at first
+int index = s2.indexOf('r'); // return 4. if not found, return -1
+```
+
+The difference between StringBuffer and StringBuilder is that the former guarantees
+thread safety. In a single-threaded environment, StringBuilder is more efficient.