String Checker II

1. String Checker II Sit-in lab 4

2. Problem Description • Given a number of strings containing only “a”, “b”, “c”, “d” • Check each string that it follows the pattern anbmcmdn where m, n >= 1 • Use only stack OR queue

3. Solution • Look at pattern: anbmcmdn • Notice that it is LIFO • So we use a Stack! • Many ways of solving with a stack… here we show one way

4. Solution • Process one character from the input String at a time. • For example: • for(int j = 0; j < s.length(); j++){strCheck.nextChar(s.charAt(j));}

5. Solution • If character is “a” • We push to stack if • the stack is empty, or • the top of the stack is “a” • Else is not valid string • If character is “b” • Remember that we have a “b” (set a boolean flag) • We push to stack if • the stack is NOT empty, and • the top of the stack is “a” or “b” • Else is not valid string We need to check this so that things like “bc” will be caught

6. Solution • If character is “c” • We pop from the stack if • the stack is NOT empty, and • the top of the stack is “b” • Else is not valid string • If character is “d” • We pop from the stack if • the stack is NOT empty, and • the top of the stack is “a”, and • there was a “b” read in (special check for things like “ad”) • Else is not valid string

7. Solution • Finally, before concluding is valid string, check that stack is empty • We need this to catch things like “aaaaaabbccd” (there will be many “a” left behind in the stack)

8. End

9. String Checker I