Added Z's Algorithm

Program's_Contributed_By_Contributors/JavaScript_Programs/z_algorithm.js

@@ -0,0 +1,90 @@
+function search(text,pattern)
+{
+    // Create concatenated string "P$T"
+        let concat = pattern + "$" + text;
+
+        let l = concat.length;
+
+        let Z = new Array(l);
+
+        // Construct Z array
+        getZarr(concat, Z);
+
+        // now looping through Z array for matching condition
+        for(let i = 0; i < l; ++i){
+
+            // if Z[i] (matched region) is equal to pattern
+            // length we got the pattern
+
+            if(Z[i] == pattern.length){
+                document.write("Pattern found at index "
+                              + (i - pattern.length - 1)+"<br>");
+            }
+        }
+}
+
+ // Fills Z array for given string str[]
+function getZarr(str,Z)
+{
+    let n = str.length;
+
+        // [L,R] make a window which matches with
+        // prefix of s
+        let L = 0, R = 0;
+
+        for(let 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.
+                let 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
+let text = "GEEKS FOR GEEKS";
+let pattern = "GEEK";
+
+search(text, pattern);