add Manacher algorithm

Manacher/Manacher.cpp

@@ -0,0 +1,50 @@
+/*  
+ *  time complexity is O(N)
+ *  The key point is cnt[i] = min(cnt[midIndex*2-i], bound-i);
+ *
+ *        i'    midIndex    i  bound
+ *        |        |        |   |
+ *  ---------------|------------******
+ * */
+
+#include <iostream>
+#include <algorithm>
+#include <vector>
+
+using namespace std;
+int manacher(string s);
+
+int main(){
+    cout << "Please input a string: "<<endl;
+    string str;
+    cin >> str;
+    cout << "The length of longest palindrome is" << manacher(str) <<endl;
+}
+
+int manacher(string s){
+    int len = s.length();
+    for(int i = 0; i <= len; ++i){
+        s.insert(i*2, "#");
+    }
+    cout << "the modified string is: " << s <<endl;
+    len = s.length();
+    vector<int> cnt(len, 1);
+    int midIndex = 0, bound = 1;   // bound is out boundary of palindrome
+    int res = 0;
+    for(int i = 1; i < len; ++i){
+        if(i < bound){
+            cnt[i] = min(cnt[2*midIndex - i], bound - i);
+        }
+
+        while(s[i - cnt[i]] == s[i + cnt[i]]){
+            ++cnt[i];
+        }
+        if(i + cnt[i] > bound)  // update the front boundary
+        {
+            midIndex = i;
+            bound = i + cnt[i];
+        }
+        res = max(res, cnt[i]);
+    }
+    return res - 1;
+}