Valid Palindrome Problem
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
Example"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.
NoteHave you consider that the string might be empty? This is a good question to ask during an interview.
For the purpose of this problem, we define empty string as valid palindrome.
重点是.toLowerCase()和Character.isLetterOrDigit()两个函数的使用;
以及指针在标点情况下移动后的continue语句。
O(n) time without extra memory. ----> 指针嘛。
Solutionpublic class Solution { public boolean isPalindrome(String s) { if (s == null || s.length() < 2) { return true; } s = s.toLowerCase(); int start = 0, end = s.length() - 1; while (start < end) { if (!Character.isLetterOrDigit(s.charAt(start))) { start++; continue; } if (!Character.isLetterOrDigit(s.charAt(end))) { end--; continue; } if (s.charAt(start) != s.charAt(end)) { return false; } start++; end--; } return true; } }Update 2018-9
class Solution { public boolean isPalindrome(String s) { if (s == null || s.length() == 0) return true; s = s.trim().toLowerCase(); int i = 0, j = s.length()-1; while (i <= j) { while (i <= j && !((s.charAt(i) >= "a" && s.charAt(i) <= "z") || (s.charAt(i) >= "0" && s.charAt(i) <= "9"))) i++; while (i <= j && !((s.charAt(j) >= "a" && s.charAt(j) <= "z") || (s.charAt(j) >= "0" && s.charAt(j) <= "9"))) j--; if (i <= j && s.charAt(i++) != s.charAt(j--)) return false; } return true; } }
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