add z-algo

C++/Algorithms/StingMatching/z_algorithm.cpp

@@ -0,0 +1,90 @@
+// A C++ program that implements Z algorithm for pattern searching
+#include<iostream>
+using namespace std;
+
+void getZarr(string str, int Z[]);
+
+// prints all occurrences of pattern in text using Z algo
+void search(string text, string pattern)
+{
+    // Create concatenated string "P$T"
+    string concat = pattern + "$" + text;
+    int l = concat.length();
+
+    // Construct Z array
+    int Z[l];
+    getZarr(concat, Z);
+
+    // now looping through Z array for matching condition
+    for (int i = 0; i < l; ++i)
+    {
+        // if Z[i] (matched region) is equal to pattern
+        // length we got the pattern
+        if (Z[i] == pattern.length())
+            cout << "Pattern found at index "
+                << i - pattern.length() -1 << endl;
+    }
+}
+
+// Fills Z array for given string str[]
+void getZarr(string str, int Z[])
+{
+    int n = str.length();
+    int L, R, k;
+
+    // [L,R] make a window which matches with prefix of s
+    L = R = 0;
+    for (int i = 1; i < n; ++i)
+    {
+        // if i>R nothing matches so we will calculate.
+        // Z[i] using naive way.
+        if (i > R)
+        {
+            L = R = i;
+
+            // R-L = 0 in starting, so it will start
+            // checking from 0'th index. For example,
+            // for "ababab" and i = 1, the value of R
+            // remains 0 and Z[i] becomes 0. For string
+            // "aaaaaa" and i = 1, Z[i] and R become 5
+            while (R<n && str[R-L] == str[R])
+                R++;
+            Z[i] = R-L;
+            R--;
+        }
+        else
+        {
+            // k = i-L so k corresponds to number which
+            // matches in [L,R] interval.
+            k = i-L;
+
+            // if Z[k] is less than remaining interval
+            // then Z[i] will be equal to Z[k].
+            // For example, str = "ababab", i = 3, R = 5
+            // and L = 2
+            if (Z[k] < R-i+1)
+                Z[i] = Z[k];
+
+            // For example str = "aaaaaa" and i = 2, R is 5,
+            // L is 0
+            else
+            {
+                // else start from R and check manually
+                L = i;
+                while (R<n && str[R-L] == str[R])
+                    R++;
+                Z[i] = R-L;
+                R--;
+            }
+        }
+    }
+}
+
+// Driver program
+int main()
+{
+    string text = "GEEKS FOR GEEKS";
+    string pattern = "GEEK";
+    search(text, pattern);
+    return 0;
+}