add longest palindromic substring

C++/LongestPalindromicSubstring.cpp

@@ -0,0 +1,47 @@
+class Solution
+{
+ string LCSubstr(string s1, string s2, vector<vector<int>> &t)
+ {
+ string ans = "";
+ int n = s1.size();
+ int mx = 0;
+ for (int i = 1; i <= n; i++)
+ {
+ for (int j = 1; j <= n; j++)
+ {
+ if (s1[i - 1] == s2[j - 1])
+ {
+ t[i][j] = 1 + t[i - 1][j - 1];
+ if (t[i][j] > mx)
+ {
+ // For every possible LC substring, we check whether it's a palindrome or not.
+ // We know the size of the LC substring, so using "i" and "t[i][j]" we can find the substring after that we just need to check whether it's palindrome or not.
+ string temp = s1.substr(i - t[i][j], t[i][j]);
+ string temp2(temp.rbegin(), temp.rend());
+
+ if (temp == temp2)
+ {
+ mx = t[i][j];
+ ans = temp;
+ }
+ }
+ }
+ else
+ {
+ t[i][j] = 0;
+ }
+ }
+ }
+
+ return ans;
+ }
+
+public:
+ string longestPalindrome(string s1)
+ {
+ int n = s1.size();
+ vector<vector<int>> t(n + 1, vector<int>(n + 1, 0));
+ string s2(s1.rbegin(), s1.rend());
+ return LCSubstr(s1, s2, t);
+ }
+};

README.md

@@ -395,6 +395,8 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 070 | [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/) | [Java](./Java/climbing-stairs.java) | _O(N)_ | _O(1)_ | Easy | DP | |
 | 730 | [Count Different Palindromic Subsequences](https://leetcode.com/problems/count-different-palindromic-subsequences/) | [C++](./C++/Count-Different-Palindromic-Subsequences.cpp) | _O(N\*N)_ | _O(N\*N)_ | Hard | DP | |
 | 55 | [Jump Game](https://leetcode.com/problems/jump-game/) | [Python](./Python/jumpGame.py) | _O(N)_ | _O(N)_ | Medium | DP | |
+| 55 | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/) | [C++](./C++/LonegstPalindromicSubstring) | _O(n^2)_ | _O(n^2)_ | Medium | DP | |
+
 
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