[LeetCode] 647. Palindromic Substrings

Problem :

Given a string, your task is to count how many palindromic substrings in this string.

The substrings with different start indexes or end indexes are counted as different substrings even they consist of same characters.

Example 1:

Input: "abc"Output: 3Explanation: Three palindromic strings: "a", "b", "c".

Example 2:

Input: "aaa"Output: 6Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".

Note:

1. The input string length won't exceed 1000.

题意分析：要求求出回文子串个数，所谓回文串，就是正序和逆序输出相同的字符串，例如"a"，"aba"，"abba"等。特别地，

题目不要求剔除相同的回文串。

解题思路：遍历字符串的所有字符，以各个字符为中心扩散查找回文子串，通过二重循环求解。

Solution:

class Solution {public:    int countSubstrings(string s) {        int count = 0;        for (int i = 0; i < s.length(); i++) {            // 奇数长度回文子串,如"a",aba","abcba"            for (int j = 0; i-j >= 0 && i+j < s.length() && s[i-j] == s[i+j]; j++)                 count++;            // 偶数长度回文子串,如"aa","abba"            for (int j = 0; i-1-j >= 0 && i+j < s.length() && s[i-1-j] == s[i+j]; j++)                count++;        }        return count;    }};

时间复杂度 = O(N^2)

解法参考自：https://discuss.leetcode.com/topic/97105/java-c-6-lines-solution-no-dp