JS solution and tests for Issue llipio#18 - Find missing number in ar…

…ray of consecutively increasing numbers

solutions/18.js

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+// Find missing number in array
+// Return the missing number in an input array of consecutively increasing numbers.
+// input: [5,6,8,9]
+// output: 7
+
+// Zach Nagatani
+
+/**
+ * Finds and returns the missing number from an array of consecutively increasing numbers
+ * @param {Number[]} arr - An array of consecutively increasing numbers
+ * @returns {Number} missingNum - the number missing from the input array
+ */
+const solution = arr => {
+    let missingNum = null,
+        i = 0;
+
+    while (!missingNum && i < arr.length - 1) {
+        const current = arr[i],
+              next = arr[i + 1];
+
+        missingNum = next === current + 1 ? missingNum : next - 1;
+        i++;
+    }
+
+    return missingNum;
+};
+
+module.exports = {
+    solution
+};

test/18.js

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+const expect = require('chai').expect;
+let solution = require('../solutions/18').solution;
+//solution = require('../yourSolution').solution;
+
+describe('Find missing number in array', () => {
+  it('should return 7 for [5,6,8,9]', () => {
+    const result = solution([5,6,8,9]);
+    expect(result).to.equal(7);
+  });
+
+  it('should return 94 for [89,90,91,92,93]', () => {
+    const result = solution([89,90,91,92,93,95]);
+    expect(result).to.equal(94);
+  });
+
+  it('should return null if there is no missing number', () => {
+    const result = solution([1,2,3,4,5]);
+    expect(result).to.equal(null);
+  });
+});