String - linear data structure

• String is immutable
• char array is mutable
• StringBuilder reduce space complexity
• Palindrome problem
• bound check
• length -1
• Edge case consideration, detail case problem
• use Integer.MAX_VALUE / 10 and Integer.MAX_VALUE % 10 to check overflow
• string is a linear structure, could use Dynamic Programming to solve optimization problems
• Regular expression matching is a DP or recursion problems, just try out all possiblilities
• brute force solution to visite each character in the string once
• s.toCharArray() return a char array char[]
• use 26 letters array freq[] to reduce space complexity to O(1)