offer2-18

给定一个字符串 s ，验证 s 是否是 回文串 ，只考虑字母和数字字符，可以忽略字母的大小写。

本题中，将空字符串定义为有效的 回文串 。

示例 1:

输入: s = "A man, a plan, a canal: Panama"
输出: true
解释："amanaplanacanalpanama" 是回文串

示例 2:

输入: s = "race a car"
输出: false
解释："raceacar" 不是回文串

提示：

• 1 <= s.length <= 2 * 105
• 字符串 s 由 ASCII 字符组成

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public boolean isPalindrome(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }

        int start = -1;
        int end = s.length();

        while (start < end) {
            start = getNextIndex(s, start, true);
            end = getNextIndex(s, end, false);

            if (start >= end) {
                break;
            }

            char startCh = s.charAt(start);
            char endCh = s.charAt(end);

            if (!isEqualChar(startCh, endCh)) {
                return false;
            }
        }

        return true;
    }

    boolean isEqualChar(char c1, char c2) {
        int i1 = -1;
        int i2 = -1;
        if ('a' <= c1 && c1 <= 'z') {
            i1 = c1 - 'a';
        }
        if ('A' <= c1 && c1 <= 'Z') {
            i1 = c1 - 'A';
        }
        if ('a' <= c2 && c2 <= 'z') {
            i2 = c2 - 'a';
        }
        if ('A' <= c2 && c2 <= 'Z') {
            i2 = c2 - 'A';
        }
        if (i1 == -1) {
            i1 = c1;
        }
        if (i2 == -1) {
            i2 = c2;
        }
        return i1 == i2;
    }

    private int getNextIndex(String s, int curIndex, boolean leftToRight) {
        if (leftToRight) {
            curIndex++;
        } else {
            curIndex--;
        }

        if (curIndex < 0) {
            return curIndex;
        }
        if (curIndex >= s.length()) {
            return curIndex;
        }

        char ch = s.charAt(curIndex);
        if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z') || ('0' <= ch && ch <= '9')) {
            return curIndex;
        }
        return getNextIndex(s, curIndex, leftToRight);
    }
}
//leetcode submit region end(Prohibit modification and deletion)