Add lowerBound implementation and unit tests

Author: lamnguyen2187

src/algorithms/search/binarysearch.js

@@ -22,4 +22,32 @@ const binarySearch = (sortedArray, element) => {
   return -1;
 };
 
-module.exports = binarySearch;
+/**
+  * lowerBound returns index of the first element greater than the search key
+  * in logarithmic time (O(log n))
+  *
+  * @param Array
+  * @param Number\String
+  *
+  * @return Number
+  */
+const lowerBound = (sortedArray, element) => {
+  let lo = 0;
+  let hi = sortedArray.length - 1;
+
+  while (lo <= hi) {
+    const mid = ((hi - lo) >> 1) + lo;
+    if (sortedArray[mid] > element) {
+      hi = mid - 1;
+    } else {
+      lo = mid + 1;
+    }
+  }
+
+  return lo;
+};
+
+module.exports = {
+  binarySearch,
+  lowerBound
+};

src/search.js

@@ -1,7 +1,8 @@
 // Search algorithms
 module.exports = {
   bfs: require('./algorithms/search/bfs'),
-  binarySearch: require('./algorithms/search/binarysearch'),
+  binarySearch: require('./algorithms/search/binarysearch').binarySearch,
+  lowerBound: require('./algorithms/search/binarysearch').lowerBound,
   ternarySearch: require('./algorithms/search/ternary_search'),
   dfs: require('./algorithms/search/dfs')
 };

src/test/algorithms/searching/binarysearch.js

@@ -1,4 +1,5 @@
 const binarySearch = require('../../..').Search.binarySearch;
+const lowerBound = require('../../..').Search.lowerBound;
 const assert = require('assert');
 
 describe('Binary Search', () => {
@@ -13,3 +14,25 @@ describe('Binary Search', () => {
     assert.equal(binarySearch([1, 2, 3, 4, 5], 100), -1);
   });
 });
+
+describe('Lower Bound', () => {
+  it('finds the first element greater than search key', () => {
+    let arr = [1, 2, 3, 4, 5];
+
+    assert.equal(lowerBound(arr, 0), 0);
+    assert.equal(lowerBound(arr, 1), 1);
+    assert.equal(lowerBound(arr, 2), 2);
+    assert.equal(lowerBound(arr, 3), 3);
+    assert.equal(lowerBound(arr, 4), 4);
+    assert.equal(lowerBound(arr, 5), 5);
+    assert.equal(lowerBound(arr, 6), 5);
+  });
+
+  it('ignores duplicate values', () => {
+    let duplicates = [1, 1, 2, 2, 3, 3];
+
+    assert.equal(lowerBound(duplicates, 1), 2);
+    assert.equal(lowerBound(duplicates, 2), 4);
+    assert.equal(lowerBound(duplicates, 3), 6);
+  });
+});