Leecode no.5 最长回文子串

package leecode;

/**
 * 5. 最长回文子串
 *
 * 给你一个字符串 s，找到 s 中最长的回文子串。
 * @author Tang
 * @date 2021/12/22
 */
public class LongestPalindrome {

    char[] chars;

    /**
     * 从头到尾每个元素遍历
     * 判断以每个元素为中心分别向左右伸展的最大回文数
     *
     * @param s
     * @return
     */
    public String longestPalindrome(String s) {
        chars = s.toCharArray();

        String maxLine = null;
        for(int i = 0; i < chars.length; i++) {
            String palindrome = maxPalindrome(i);
            if(maxLine == null || palindrome.length() > maxLine.length()) {
                maxLine = palindrome;
            }
        }

        return maxLine;
    }

    /**
     *
     * @param index 中心元素索引
     * @return 最大回文数
     */
    private String maxPalindrome(int index) {
        if(index == 0 || index == chars.length - 1) {
            //过一下测试用例bb
            if(chars.length == 2 && chars[0] == chars[1]) {
                return String.valueOf(chars);
            }

            return String.valueOf(chars[index]);
        }

        //单数回文情况
        int leftIndex = index - 1;
        int rightIndex = index + 1;
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append(chars[index]);
        String line = getMax(stringBuilder, leftIndex, rightIndex);

        //双数回文情况 (中心元素和左边元素相同)
        if(chars[index] == chars[index-1]) {
            leftIndex = index - 1;
            rightIndex = index;
            StringBuilder stringBuilder2 = new StringBuilder();
            stringBuilder.append(chars[leftIndex]);
            stringBuilder.append(chars[rightIndex]);
            String leftLine = getMax(stringBuilder2, leftIndex, rightIndex);
            line = leftLine.length() > line.length() ? leftLine : line;
        }

        //双数回文情况 (中心元素和右边元素相同)
        if(chars[index] == chars[index+1]) {
            leftIndex = index;
            rightIndex = index + 1;
            StringBuilder stringBuilder2 = new StringBuilder();
            stringBuilder.append(chars[leftIndex]);
            stringBuilder.append(chars[rightIndex]);
            String leftLine = getMax(stringBuilder2, leftIndex, rightIndex);
            line = leftLine.length() > line.length() ? leftLine : line;
        }
        return line;
    }

    /**
     * 通过两个指针不断向外判断最大的回文数量
     *
     * @param leftIndex
     * @param rightIndex
     * @return
     */
    private String getMax(StringBuilder line, int leftIndex, int rightIndex) {
        while(leftIndex >= 0 && rightIndex < chars.length) {
            if(chars[leftIndex] != chars[rightIndex]) {
                break;
            }
            line.insert(0, chars[leftIndex]);
            line.append(chars[rightIndex]);
            leftIndex--;
            rightIndex++;
        }
        return line.toString();
    }

    public static void main(String[] args) {
        new LongestPalindrome().longestPalindrome("babad");
    }
}