add longestPalindrome.cpp

Author: kamyu104

C++/longestPalindrome.cpp

@@ -0,0 +1,52 @@
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+class Solution {
+    public:
+        // Manacher's Algorithm
+        string longestPalindrome(string s) {
+            string T = preProcess(s);
+            int n = T.length();
+            vector<int> P(n);
+            int C = 0, R = 0;
+            for (int i = 1; i < n-1; i++) {
+                int i_mirror = 2*C-i; // equals to i' = C - (i-C)
+
+                P[i] = (R > i) ? min(R-i, P[i_mirror]) : 0;
+
+                // Attempt to expand palindrome centered at i
+                while (T[i + 1 + P[i]] == T[i - 1 - P[i]])
+                    P[i]++;
+
+                // If palindrome centered at i expand past R,
+                // adjust center based on expanded palindrome.
+                if (i + P[i] > R) {
+                    C = i;
+                    R = i + P[i];
+                }
+            }
+
+            // Find the maximum element in P.
+            int maxLen = 0;
+            int centerIndex = 0;
+            for (int i = 1; i < n-1; i++) {
+                if (P[i] > maxLen) {
+                    maxLen = P[i];
+                    centerIndex = i;
+                }
+            }
+
+            return s.substr((centerIndex - 1 - maxLen)/2, maxLen);
+        }
+    private:
+        string preProcess(string s) {
+            int n = s.length();
+            if (n == 0) return "^$";
+            string ret = "^";
+            for (int i = 0; i < n; i++)
+                ret += "#" + s.substr(i, 1);
+
+            ret += "#$";
+            return ret;
+        }
+};