Adding Program to compute Longest Common Leading and Trailing Substring with tests cases

.gitignore

@@ -3,3 +3,4 @@
 /.nyc_output
 /npm-debug.log
 dist
+**/.DS_Store

src/algorithms/string/LongestCommLeadtrailSubString.js

@@ -0,0 +1,45 @@
+/**
+ * Calculates Longest common leading and trailing substring of two numbers
+ * @param  {Number} a First string
+ * @param  {Number} b Second string
+ * @return {Number}   Longest common leading/trailing substring
+ */
+const LongestCommLeadtrailSubString = (str1, str2) => {
+        var leading = "";
+        var trailing = "";
+        var leadflag = false;
+        var trailflag = false;
+        if(str1.length ===0 || str2.length === 0){
+                console.log("Must enter valid strings")
+        }
+
+        var end1 = str1.length - 1
+        var end2 = str2.length - 1
+
+        let pos = 0
+
+        while (pos <= end1 && pos <= end2 && (leadflag === false || trailflag === false)) {
+                if (leadflag === false) {
+                        if (str1[pos] === str2[pos]) {
+                                leading = str1.substring(0, pos+1)
+
+                        }
+                        else{
+                                leadflag = true
+                        }
+                }
+                if (trailflag === false) {
+                        if (str1[end1 - pos] === str2[end2 - pos]) {
+                                trailing = str1.substring(end1 - pos)
+
+                        }
+                        else{
+                                trailflag = true;
+                        }
+                }
+                pos++
+        }
+        return {"leading":leading,"trailing": trailing}
+};
+
+module.exports = LongestCommLeadtrailSubString;

test/algorithms/string/testleadtrail.js

@@ -0,0 +1,38 @@
+/* eslint-env mocha */
+const LongestCommLeadtrailSubString = require('../../../src/algorithms/string/LongestCommLeadtrailSubString');
+const expect = require('expect');
+
+
+describe('Longest common leading and trailing substring of two strings', () => {
+  it('should return empty strings if the inputs mismatch entirely', () => {
+    const stringA = 'ABCDEF';
+    const stringB = 'QRST';
+
+    const SubStrings = LongestCommLeadtrailSubString(stringA, stringB);
+    expect(SubStrings).toStrictEqual({"leading":"","trailing": ""});
+  });
+
+  it('should return only trailing common substring if no leading common subtring found', () => {
+    const stringA = 'ABCMNOP';
+    const stringB = 'EFGHMNOP';
+
+    const SubStrings = LongestCommLeadtrailSubString(stringA, stringB);
+    expect(SubStrings).toStrictEqual({"leading":"","trailing": "MNOP"});
+  });
+
+  it('should return only leading common substring if no trailing common subtring found', () => {
+    const stringA = 'ABCMNOP';
+    const stringB = 'ABCYZ';
+
+    const SubStrings = LongestCommLeadtrailSubString(stringA, stringB);
+    expect(SubStrings).toStrictEqual({"leading":"ABC","trailing": ""});
+  });
+
+  it('should return both leading common substring and trailing common subtring if found', () => {
+    const stringA = 'ABCYEFG';
+    const stringB = 'ABCZEFG';
+
+    const SubStrings = LongestCommLeadtrailSubString(stringA, stringB);
+    expect(SubStrings).toStrictEqual({"leading":"ABC","trailing": "EFG"});
+  });
+});