Added prefix function

Author: sslotin

prefix_function.cpp

@@ -0,0 +1,12 @@
+vector<int> z_function (string s) {
+    int n = (int) s.size();
+    vector<int> p(n, 0);
+    for (int i = 1; i < n; i++) {
+        int cur = p[i - 1];
+        while (s[i] != s[cur] && cur > 0)
+            cur = p[cur - 1];
+        if (s[i] == s[cur])
+            p[i] = cur + 1;
+    }
+    return p;
+}