p89 回文子串个数(leetcode 647)

一：解题思路

以每个字符串中的字符为中分别向两边进行扩展，然后对比是否相等。比如：aba 有4个回文字符串。abba有6个回文字符串。Time:O(n^2),Space:O(1)

二：完整代码示例 (C++版和Java版)

C++:

class Solution
{
public:
    int expand(string s, int left, int right)
    {
        int count = 0;
        while (left >= 0 && right < s.size() && s[left] == s[right])
        {
            count++;
            left--;
            right++;
        }

        return count;
    }

    int countSubstrings(string s)
    {
        if(s.size()==0) return 0;
        int count = 0;
        for (int i = 0; i < s.size(); i++)
        {
            count += expand(s,i,i);
            count += expand(s,i,i+1);
        }

        return count;
    }
};

Java:

class Solution {
        private int expand(String s,int left,int right)
        {
            int count=0;
            while(left>=0 && right<s.length() && s.charAt(left)==s.charAt(right))
            {
                count++;
                right++;
                left--;
            }
            
            return count;
        }   
        
        public int countSubstrings(String s)
        {
              if(s==null || s.length()==0) return 0;
              int count=0;
              for(int i=0;i<s.length();i++)
              {
                  count+=expand(s,i,i);
                  count+=expand(s,i,i+1);
              }
              
              return count;
        }
    }