Leetcode 647: Palindromic Substrings

Posted 2020-10-22 Keep walking

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.

 

 

 1 public class Solution {
 2     public int CountSubstrings(string s) {
 3         if (s == null || s.Length == 0) return 0;
 4         
 5         var dp = new bool[s.Length, s.Length];
 6         
 7         int count = 0;
 8         
 9         for (int i = s.Length - 1; i >= 0; i--)
10         {
11             for (int j = i; j < s.Length; j++)
12             {
13                 if ((s[i] == s[j]) && (j - i <= 2  || dp[i + 1, j - 1]))
14                 {
15                     dp[i, j] = true;
16                     count++;
17                 }
18             }
19         }
20         
21         return count;
22     }
23 }
24 
25 public class Solution1 {
26     public int CountSubstrings(string s) {
27         if (s == null || s.Length == 0) return 0;
28         
29         int count = 0;
30         var dict = new Dictionary<Tuple<int, int>, bool>();
31         
32         for (int i = s.Length - 1; i >= 0; i--)
33         {
34             for (int j = i; j < s.Length; j++)
35             {
36                 if (IsPalindrome(s, i, j, dict)) count++;    
37             }
38         }
39         
40         return count;
41     }
42     
43     private bool IsPalindrome(string s, int i, int j, Dictionary<Tuple<int, int>, bool> dict)
44     {
45         var key = new Tuple<int, int>(i, j);
46         
47         if (i == j)
48         {
49             dict[key] = true;
50             return true;
51         }
52         
53         if (i + 1 == j)
54         {
55             if (s[i] == s[j])
56             {
57                 dict[key] = true;
58                 return true;
59             }
60             else
61             {
62                 dict[key] = false;
63                 return false;
64             }
65         }
66         
67         var key1 = new Tuple<int, int>(i + 1, j - 1);
68         
69         // since we have aleady processed them before, we don‘t need to process again
70         if (s[i] == s[j] && dict.ContainsKey(key1) && dict[key1])
71         {
72             dict[key] = true;
73             return true;
74         }
75         else
76         {
77             dict[key] = false;
78             return false;
79         }
80     }
81 }