leetcode(5)最长回文子串

最长回文子串

解题思想：动态规划

class Solution {
    public String longestPalindrome(String s) {
        int len = s.length();
        boolean[][] b = new boolean[len][len];
        int max = 0;
        int begin = 0;
        for(int i=0;i<len;i++){
            for(int j=0;j<=i;j++){
                if(s.charAt(i)==s.charAt(j)&&(i-j<=2 || b[j+1][i-1])){
                    b[j][i] = true;
                    if(i-j+1>max){
                        max = i - j + 1;
                        begin = j;
                    }
                }
            }
        }
        return s.substring(begin,begin+max);
    }
}

马拉车算法：

解题思想：中心扩散法

    public static String longestPalindrome(String s) {
        int len = s.length();
        if(len == 0) {
            return "";
        }
        StringBuilder strD = new StringBuilder();
        strD.append('*');
        for(int i=0;i<len;i++) {
            strD.append('#');
            strD.append(s.charAt(i));
        }
        strD.append('#');
        int slen = strD.length();
        int[] r = new int[slen];
        int mx = 0;
        int id = 0;
        int rmax = 0;
        int meduim = 0;
        for(int i=0;i<slen;i++) {
            r[i]= mx>i?Math.min(r[2*id-i],mx-i):1;
            while(i-r[i]>=0&&i+r[i]<slen&&strD.charAt(i-r[i])==strD.charAt(i+r[i])) {
                r[i]++;
            }
            if(mx<i+r[i]) {
                mx=i+r[i];
                id = i;
            }
            if(r[i]>rmax) {
                rmax = r[i];
                meduim = i;
            }
        }
        //return strD.substring(meduim-rmax+1,meduim+rmax).replace("#", "");
        int start = (meduim-rmax)/2;
        return s.substring(start,start+rmax-1);
    }    

 参考文献：

马拉车算法：https://blog.csdn.net/Dby_freedom/article/details/93191052