How to compare two strings while some parts of them are same?
Let say I have a string `ABCAAAA`. 

For some reason, only ONE character of the string `ABCAAAA` can be changed at a time. For example, I can change this string to `DBCAAAA`.

Now the problem is :
How can I ensure **ONLY ONE** character is changed each time? Is there a method for `NSString` to compare how much two strings differ?

**Purpose**: I put each string into own `UITextField` to determine whether this one is **editable** if others had changed. I need to ensure only one is edited at a time. So if one had been edited, I will set `UITextField`'s `enable` to NO to disable editing.