设计算法，判断该字符串的前n个字符组成的是否为回文，且使算法时间复杂度最小。

（注：123321、abcdeedcba均为回文，但12321不是回文）

-Design algorithm to determine the first n characters of the string is a palindrome, and the complexity of the algorithm time minimum. (Note: 123321, abcdeedcba are palindromes, but 12321 is not a palindrome)