2020.7.12 5. 最长回文子串

 

 

5. 最长回文子串

思想：动态规划

dp[i][j] 表示从 i 到 j 是否为回文子串。

dp[i][j] = (s[i] == s[j]) && dp[i + 1][j - 1], 即判断s[i]和s[j]是否相等，相等判断i+1到j-1是否为回文子串，若是则 i 到 j 也是回文子串。

代码实现

import java.util.*;

public class Solution {
    public String longestPalindrome(String s) {
        int n = s.length();
        boolean[][]dp = new boolean[n][n];
        for (int i = 1; i < n; i++) {
            dp[i][i] = true;
            dp[i][i-1] = true;
        }
        dp[0][0] = true;
        int posx = 0, posy = 0, max = 0;
        for (int j = 1; j < n; j++) {
            for (int i = 0; i < j ; i++) {
                if(dp[i+1][j-1] && s.charAt(i) == s.charAt(j)){
                    dp[i][j] = true;
                    if (Math.abs(i-j) + 1 > max){
                        max  = Math.abs(i-j) + 1;
                        posx = i;
                        posy = j;
                    }
                }
                else dp[i][j] = false;
            }
        }
        return s.substring(posx, posy + 1);
    }

    public static void main(String[] args) {
        Solution s = new Solution();
        Scanner in = new Scanner(System.in);
        System.out.println(s.longestPalindrome("cbbd"));
    }
}

6. Z 字形变换

思路：

public String convert(String s, int numRows) {
        if(s.length() <= 2) return s;
        if(numRows == 1) return s;
//        List<StringBuffer> list = new ArrayList<>();
        StringBuffer[] stb = new StringBuffer[numRows];

        for (int i = 0; i < numRows; i++) {
            stb[i] = new StringBuffer();
        }
        int i = 0, flag = -1;
        for (char op : s.toCharArray()) {
            stb[i].append(op);
            if(i == 0 || numRows - 1 == i) flag = -flag;
            i += flag;
        }
        StringBuffer res = new StringBuffer();
        for (int j = 0; j < numRows; j++) {
            res.append(stb[j]);
        }
        return new String(res);
    }