Solution and tests for issue 19, find pairs that sum to x

solutions/19.js

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+// Find pairs that sum to x
+// Return array of pairs from an input array of numbers that adds up to second input x. Each number can be used only once.
+// input: [1,2,3,4,5], 6
+// output: [[1,5], [2,4]]
+
+// Solution by Zach Nagatani @zachnagatani
+
+/**
+ * Returns an multidimensional array containing pairs of numbers from an passed in array which add up to the passed in sum
+ * Each integer can only be used to form one pair
+ * @param {number[]} arr - An array of numbers to search through
+ * @param {number} sum - The sum to use as the basis of the search
+ * @returns {number[][]} pairs - A multidimensional array of pairs that add up to the sum
+ */
+const solution = (arr, sum) => {
+    let unusedNums = arr.slice(),
+        pairs = [],
+        head = 0,
+        tail = unusedNums.length - 1;
+
+    while (head < tail) {
+        let currentSum = unusedNums[head] + unusedNums[tail];
+
+        if (currentSum === sum) {
+            pairs.push([unusedNums[head], unusedNums[tail]]);
+            unusedNums.splice(head, 1);
+            unusedNums.splice(tail, 1);
+
+            head = 0;
+            tail = unusedNums.length - 1;
+        } else if (currentSum < sum) {
+            head++;
+        } else {
+            tail--;
+        }
+    }
+
+    return pairs.length ? pairs : null;
+};
+
+module.exports = {
+    solution
+};

test/19.js

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+const expect = require('chai').expect;
+let solution = require('../solutions/19').solution;
+//solution = require('../yourSolution').solution;
+
+describe('Array pairs that sum to x', () => {
+    it('should return an array of pairs of integers whose sum equal to x', () => {
+        const result = solution([1,2,3,4,5,6], 6);
+        expect(result).to.deep.equal([[1,5], [2,4]]);
+
+        const result2 = solution([2,1,3,4,5,6], 6);
+        expect(result).to.deep.equal([[1,5], [2,4]]);
+    });
+
+    it('should only use an integer once in its output', () => {
+        const result = solution([1,3,3,3], 4);
+        expect(result).to.deep.equal([[1,3]]);
+    });
+
+    it('should return null of no pairs are found', () => {
+        const result = solution([1,2,3,4,5,6], 40);
+        expect(result).to.equal(null);
+    });
+});