字符串的回文子串数量

leetcode 647. Palindromic Substrings

方法一：中心回文法

class Solution {
public:
	int countSubstrings(string s) 
	{
		int n = s.length();
		int ans = 0;
		for (int i = 0; i < n;i++)
		{
			for (int j = 0; i + j < n && i - j>=0 && s[i - j] == s[i + j]; j++) ans++;
			for (int j = 0; i + j < n && i - 1 - j>=0 && s[i - 1 - j] == s[i + j]; j++) ans++;
		}
		return ans;
	}
};

方法二：动态规划法

class Solution {
public:
	int countSubstrings(string s) 
	{
		int n = s.length();
		if (n==0)
		{
			return 0;
		}
		int ans = 0;
		vector<vector<int>> dp(n, vector<int>(n, 0));
		for (int i = n - 1; i >= 0;i--)
		{
			for (int j = i; j < n;j++)
			{
				dp[i][j] = ((s[i] == s[j]) && (j - i < 3 || dp[i + 1][j - 1]));
				if (dp[i][j])
				{
					ans++;
				}
			}
		}
		return ans;
	}
};