leetcode[5]Longest Palindromic Substring(最长回文子串)

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example:

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.

Example:

Input: "cbbd"

Output: "bb"

---

Solution1(DP)

public class Solution {
    public String longestPalindrome(String s) {
        //DP
        if(s == null || s.isEmpty()){
            return "";
        }
        int n = s.length() ;
        int maxI = 0 ;
        int maxJ = 0 ;
        boolean [][] isPa = new boolean [n][n];
        for(int i = 0 ; i < n ; i ++){
            isPa [i][i] = true ;
            if((i + 1 ) < n && s.charAt(i) == s.charAt(i + 1)){
                isPa[i][i + 1] = true ;
                maxI = i ;
                maxJ = i + 1 ;
            }
        }
        
        //assume l >= 3
        for(int l = 3 ; l <= n ; l ++){
            for(int i = 0 ; i < n - l + 1 ; i ++){//i < n - l + 1
                int j  = i + l - 1;//pay attention to array's bound
                isPa[i][j] = (isPa[i + 1 ][j - 1] && s.charAt(i) == s.charAt(j));//字符串[i,j]是回文的当且仅当[i+1,j-1]是回文的且字符i等于字符j
                if(isPa[i][j]){
                    maxI = i ;
                    maxJ = j ;
                }
            }
        }
        return s.substring(maxI , maxJ + 1);
    }
}

Solution2:(往回文字符串的两边拓展寻找更长的回文字符串)

public class Solution {
    public String longestPalindrome(String s) {
		if (s == null) {
			return "";
		}
		char[] arr = s.toCharArray();
		int max = 0;
		int maxi = 0;
		int maxj = 0;

        int len = arr.length;

		for (int i = 0; i < len;) {
			int i1 = getFarestSameElementIndex(arr, i,len);
			int dist = getDistance(arr, i, i1,len);
			int index1 = i - dist;
			int index2 = i1 + dist;
			int l = index2 - index1;
			if (l > max) {
				max = l;
				maxi = index1;
				maxj = index2;
			}
			i = i1 + 1;//jump over the same chars 
		}

		return s.substring(maxi, maxj + 1);
	}

        //以[index1,index2]作为已经是回文的字符串，想两边拓展，返回最大拓展距离
	private int getDistance(char[] arr, int index1, int index2,int len) {
		int i1 = index1 - 1;
		int i2 = index2 + 1;
		int dist = 0;
		while (i1 >= 0 && i2 < len) {
			if (arr[i1] == arr[i2]) {
				dist++;
			} else {
				break;
			}
			i1--;
			i2++;
		}
		return dist;
	}

        //返回index开始的相同字符的最大长度
	private int getFarestSameElementIndex(char[] arr, int index,int len) {
		for (int i = index + 1; i < len; i++) {
			if (arr[i] != arr[index]) {
				return i - 1;
			}
		}
		return len - 1;
	}
}