Merge pull request #3 from divi-259/lc

divi-259 · web-flow · commit d3eeade69b19 · 2023-10-27T20:02:13.000+05:30
Oct 27 Daily LC Longest Palindromic substring
diff --git a/Daily LC/27_Oct_2023_Longest_Palindromic_Substring.java b/Daily LC/27_Oct_2023_Longest_Palindromic_Substring.java
@@ -0,0 +1,48 @@
+/*
+Problem Link: https://leetcode.com/problems/longest-palindromic-substring/description/
+
+Problem Statement: Given a string s, return the longest palindromic substring in s.
+
+Solution Approach: 
+For every index i in the string, have an external function expandfromMiddle to return the longest length of palindrome that
+can be formed as this index in the middle.
+Also keep in mind two kinds of test cases where palindrome can be of even or odd length.
+
+*/
+
+/* ------------CODE---------------- */
+
+class Solution {
+    public String longestPalindrome(String s) {
+        int len = s.length();
+        if(len<2)
+        return s;
+        int start = 0;
+        int end = 0;
+        for(int i=0; i<s.length(); i++) {
+            int len1 = expandFromMiddle(s, i, i);  // racecar example, checking for e==e
+            int len2 = expandFromMiddle(s, i, i+1);
+            int lenz = Math.max(len1,len2);
+            if(lenz > end-start) {
+                start = i - ((lenz-1)/2);
+                end = i + (lenz/2);
+            }
+        }
+        return s.substring(start, end+1);
+    }
+
+
+    public int expandFromMiddle(String s, int left, int right) {
+        if(s==null || left>right)
+            return 0;
+        while(left>=0 && right<s.length() && s.charAt(left)==s.charAt(right)) {
+            left--;
+            right++;
+        }
+        return right-left-1;
+    }
+}
+/*
+Time Complexity: O(n^2)
+Space Complexity: O(1) - we do not need any extra space
+*/
