最长对称子字符串

因为给定一个字符串，只含数字或大小写字母，可以在每相邻字符之间添加'*'，就不用分aba和abba两种情况了。

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();
        // 在每相邻两个字符之间添加‘*’
        str = str.replaceAll(".", "*$0").substring(1);
        int maxArea = 0, maxIndex = 0;
        for (int i = 0; i < str.length(); i++) {
            int temp = getArea(str, i);
            if (temp > maxArea) {
                maxArea = temp;
                maxIndex = i;
            }
        }
        System.out.println(str.substring(maxIndex - maxArea, maxIndex + maxArea + 1).replace("*", ""));
    }

    private static int getArea(String str, int index) {
        int area = 0;
        int left = index - 1, right = index + 1;
        while (left >= 0 && right <= str.length() - 1) {
            // 向左右两边扩展
            if (str.charAt(left--) == str.charAt(right++)) {
                area++;
            } else {
                break;
            }
        }
        return area;
    }
}

	import java.util.Scanner;

public class Main{

    public static void main(String args[]) {
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        String symmetryPart = getSymmetryString(input);
        System.out.println(symmetryPart);

    }

    // 有两种对称
    // 一种（way1str）：baab式；
    // 一种(way2str)：bab式；
    public static String getSymmetryString(String str) {
        String way1str = getSymmetryStringWay1(str);
        String way2str = getSymmetryStringWay2(str);
        // 谁长选谁
        return (way1str.length() > way2str.length()) ? way1str : way2str;

    }

    // bab式；
    public static String getSymmetryStringWay2(String str) {
        char[] chars = str.toCharArray();
        int repeatTime = 0;
        int centerIndex = 0;
        for (int i = 0; i < chars.length - 2; i++) {
            if (chars[i] == chars[i + 2]) {
                int repeatTemp = getSymmetryQuantityWay2(chars, i + 1);
                if (repeatTemp > repeatTime) {
                    repeatTime = repeatTemp;
                    centerIndex = i + 1;
                }
            }
        }
        return str.substring(centerIndex - (repeatTime - 1) / 2, centerIndex
                + (repeatTime - 1) / 2 + 1);
    }

    // baab式；
    public static String getSymmetryStringWay1(String str) {
        char[] chars = str.toCharArray();

        int repeatTime = 0;
        int repeatIndex = 0;
        for (int i = 0; i < chars.length - 1; i++) {
            if (chars[i] == chars[i + 1]) {
                int repeatTemp = getSymmetryQuantityWay1(chars, i);
                if (repeatTemp > repeatTime) {
                    repeatTime = repeatTemp;
                    repeatIndex = i;
                }
            }
        }
        return str.substring(repeatIndex - repeatTime / 2 + 1, repeatIndex
                + repeatTime / 2 + 1);

    }

    // bab式；
    private static int getSymmetryQuantityWay2(char[] chars, int i) {
        int repeat = 1;
        int indexLeft = i - 1;
        int indexRight = i + 1;
        while (chars[indexLeft] == chars[indexRight]) {
            indexLeft--;
            indexRight++;
            repeat += 2;
            if (indexLeft < 0 || indexRight > chars.length - 1) {
                return repeat;
            }
        }
        return repeat;

    }

    // baab式；
    private static int getSymmetryQuantityWay1(char[] chars, int i) {
        int repeat = 0;
        int indexLeft = i;
        int indexRight = i + 1;
        while (chars[indexLeft] == chars[indexRight]) {
            indexLeft--;
            indexRight++;
            repeat += 2;
            if (indexLeft < 0 || indexRight > chars.length - 1) {
                return repeat;
            }
        }
        return repeat;
    }

}
	

		
	
//开始以为只有abba式，结果不能完全通过，增加了aba式就通过了。代码有很多重复的地方，应该重构一下。

public class Main{ public static boolean ispalindrome(String sub) { for(int i=0; i < sub.length()/2; i++)
        { if(sub.charAt(i) != sub.charAt(sub.length() - i - 1)) return false;
        } return true;
    } public static void main(String[] args) throws IOException {
        BufferedReader buf = new BufferedReader(new InputStreamReader(System.in));
        String str = buf.readLine();
        String palin_str = str.substring(0,1); for (int i = 0; i < str.length(); i++) { for (int j = i + 1; j < str.length() + 1; j++) { if (ispalindrome(str.substring(i, j))){ if(str.substring(i, j).length() > palin_str.length())
                        palin_str = str.substring(i,j);
                }
            }
        }
        System.out.println(palin_str);
    }
}