Longest_Common_Subsequence and Rabin_karp

src/algorithms/string/Longest_Common_Subsequence.js

@@ -0,0 +1,43 @@
+
+const Longest_Common_Subsequence = (arr1, arr2) => {
+
+    let LCS_def = Array.from({ length: arr1.length + 1 },
+        () => Array.from({ length: arr2.length + 1 },
+            () => 0));
+
+    for (let i = 0; i < arr1.length + 1; i++){
+        for (let j = 0; j < arr2.length+1; j++){
+
+            if (i == 0 || j == 0) {
+                LCS_def[i][j] = 0;
+            }
+
+            // when the last character of both match, increase length of lcs by 1
+            else if (arr1[i - 1] == arr2[j - 1]) {
+                LCS_def[i][j] = LCS_def[i - 1][j - 1] + 1;
+            }
+
+            /* 
+            when the last character is not same, take maximum obtained by adding 
+            one character to one of the subsequences.
+            */
+            else {
+                LCS_def[i][j] = Math.max(LCS_def[i - 1][j], LCS_def[i][j - 1])
+            }
+        }
+    }
+
+    return `The length of longest subsequence is: ${LCS_def[arr1.length][arr2.length]}`;
+
+}
+
+  module.exports = Longest_Common_Subsequence;
+
+// I/P and O/P Examples
+
+const inputSet1 = [10, 15, 20, 25, 30, 35, 40];
+const inputSet2 = [10, 12, 23, 25, 28, 30, 32, 40];
+const result = Longest_Common_Subsequence(inputSet1, inputSet2);
+
+// The length of longest subsequence is: 4
+console.log(result);

src/algorithms/string/Rabin_karp.js

@@ -0,0 +1,43 @@
+const rabinKarp = (pat, str) => {
+    let p = 31
+    let m = 1000000007
+    let S = pat.length
+    let T = str.length
+
+    let p_pow = [] // precomputing powers of p mod m 
+    p_pow.push(1) // p^0 = 1
+    for (let i = 1; i < Math.max(S, T); i++) {
+        p_pow.push((p_pow[i - 1] * p) % m);
+    }
+
+    let h = [] // length-wise hash i.e h[i] = hash of the prefix with i characters
+    h.push(0); // h[0] = 0   
+    for (let i = 0; i < T; i++) {
+        h.push((h[i] + (str[i].charCodeAt(0) - 97 + 1) * p_pow[i]) % m);
+    }
+
+    let h_s = 0; // hash value of the pattern
+    for (let i = 0; i < S; i++)
+        h_s = (h_s + (pat[i].charCodeAt(0) - 97 + 1) * p_pow[i]) % m;
+
+    let occurs = []; // desired array storing the indices of given string where pattern occurs
+    for (let i = 0; i + S - 1 < T; i++) {
+        // slide a window of length S of the pattern
+        let cur_h = (h[i + S] - h[i] + m) % m;
+        // if current_hash matches with the hash of out pattern, then a match is found at this index
+        if (cur_h == h_s * p_pow[i] % m)
+            occurs.push(i);
+    }
+
+    return occurs;
+}
+
+// Calling the function
+let str = "girlscriptsummerofcodewithgirlsandboys"
+let pat = "girl"
+
+let ids = rabinKarp(pat, str) // desired array storing the indices of given string where pattern occurs
+console.log("Pattern found at indexes: ", ids)
+module.export = rabinKarp;
+// Output
+// Pattern found at indexes:  [ 0, 26 ]

src/algorithms/string/index.js

@@ -1,5 +1,9 @@
 const levenshteindistance = require('./levenshtein_distance');
+const Longest_Common_Subsequence = require('./Longest_Common_Subsequence');
+const rabinKarp = require('./Rabin_karp');
 
 module.exports = {
-  levenshteindistance
+  levenshteindistance,
+  Longest_Common_Subsequence,
+  rabinKarp,
 };