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Thursday, July 27, 2017

[Leetcode] Palindromic Substrings, Solution

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: 3
Explanation: Three palindromic strings: "a", "b", "c".
Example 2:
Input: "aaa"
Output: 6
Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".
Note:
  1. The input string length won't exceed 1000.

[Thoughts]
从左到右遍历字符串,以每一个字符为中心,向左右两边扩展扫描,统计palindromic string。

另外,这题也可以用DP,类似于 http://fisherlei.blogspot.com/2012/12/leetcode-longest-palindromic-substring.html

[Code]
1:    int countSubstrings(string s) {  
2:      if(s == "") return 0;  
3:        
4:      int count = 1;   
5:      for(int i =1; i< s.size(); i++) {  
6:        // palindrom length is odd and i in the middle  
7:        count += countPalindrom(s, i, i);  
8:          
9:        // palindrom length is even and (i-1, i) in themiddle  
10:        count += countPalindrom(s, i-1, i);  
11:      }  
12:      return count;  
13:    }  
14:      
15:    int countPalindrom(string& s, int start, int end) {  
16:      int count = 0;  
17:      for(int i =0; start >=0 && end <s.size(); i++)  
18:      {  
19:        if(s[start] != s[end]) break;  
20:          
21:        count++;  
22:        start--;  
23:        end++;  
24:      }  
25:        
26:      return count;  
27:    }  
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