125 验证回文串

本题中，我们将空字符串定义为有效的回文串

输入: "A man, a plan, a canal: Panama"
输出: true

输入: "race a car"
输出: false

public boolean isPalindrome(String s) {
         if (s == null || s.length() == 0) {
            return true;
        }

        StringBuilder sb = new StringBuilder();

        char[] chars = s.toCharArray();

        for (int i = 0; i < chars.length; i++) {
            if ((chars[i] >= 'a' && chars[i] <= 'z') || (chars[i] >= 'A' && chars[i] <= 'Z') || (chars[i] >= '0' && chars[i] <= '9')) {
                sb.append(chars[i]);
            }
        }

        chars = sb.toString().toCharArray();
        for (int i = 0, j = chars.length - 1; i <= chars.length / 2 - 1; i++, j--) {
            if (Character.toUpperCase(chars[i]) != Character.toUpperCase(chars[j])) {
                return false;
            }
        }
        return true;
    }

从数组两端同时开始遍历，比较是否相等