代买随想录二刷day57

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前言

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一、力扣647. 回文子串

class Solution {
    public int countSubstrings(String s) {
        int n = s.length();
        boolean[][] dp = new boolean[n][n];
        int res = 0;
        for(int i = n-1; i >= 0; i --){
            for(int j = i; j < n; j ++){
                if(s.charAt(i) == s.charAt(j)){
                    if(j -i <= 1){
                        res ++;
                        dp[i][j] = true;
                    }else if(dp[i+1][j-1]){
                        res ++;
                        dp[i][j] = true;
                    }
                }
            }
        }
        return res;
    }
}

class Solution {
    public int countSubstrings(String s) {
        int res = 0;
        for(int i = 0; i < s.length(); i ++){
            res += fun(i,i,s);
            res += fun(i,i+1,s);
        }
        return res;
    }
    public int fun(int start, int end, String s){
        int res = 0;
        while(start >= 0 && end < s.length()){
            if(s.charAt(start) != s.charAt(end)){
                return res;
            }
            start --;
            end ++;
            res ++;
        }
        return res;
    }
}

二、力扣516. 最长回文子序列

class Solution {
    public int longestPalindromeSubseq(String s) {
        int n = s.length();
        int[][] dp = new int[n][n];
        for(int i = n-1; i >= 0; i --){
            for(int j = i; j < n; j ++){
                if(s.charAt(i) == s.charAt(j)){
                    if(j - i <= 1){
                        dp[i][j] = j-i+1;
                    }else{
                        dp[i][j] = dp[i+1][j-1] + 2;
                    }
                }else{
                    dp[i][j] = Math.max(dp[i][j-1], dp[i+1][j]);
                }
            }
        }
        return dp[0][n-1];
    }  
}