Add solution #39

Author: JoshCrozier

README.md

@@ -39,6 +39,7 @@
 34|[Find First and Last Position of Element in Sorted Array](./0034-find-first-and-last-position-of-element-in-sorted-array.js)|Medium|
 35|[Search Insert Position](./0035-search-insert-position.js)|Easy|
 36|[Valid Sudoku](./0036-valid-sudoku.js)|Medium|
+39|[Combination Sum](./0039-combination-sum.js)|Medium|
 41|[First Missing Positive](./0041-first-missing-positive.js)|Hard|
 42|[Trapping Rain Water](./0042-trapping-rain-water.js)|Hard|
 43|[Multiply Strings](./0043-multiply-strings.js)|Medium|

solutions/0039-combination-sum.js

@@ -0,0 +1,39 @@
+/**
+ * 39. Combination Sum
+ * https://leetcode.com/problems/combination-sum/
+ * Difficulty: Medium
+ *
+ * Given an array of distinct integers candidates and a target integer target,
+ * return a list of all unique combinations of candidates where the chosen
+ * numbers sum to target. You may return the combinations in any order.
+ *
+ * The same number may be chosen from candidates an unlimited number of times.
+ * Two combinations are unique if the frequency of at least one of the chosen
+ * numbers is different.
+ *
+ * It is guaranteed that the number of unique combinations that sum up to target
+ * is less than 150 combinations for the given input.
+ */
+
+/**
+ * @param {number[]} candidates
+ * @param {number} target
+ * @return {number[][]}
+ */
+var combinationSum = function(candidates, target) {
+  const result = [];
+  backtrack(result, candidates, target);
+  return result;
+};
+
+function backtrack(result, candidates, target, combination = [], offset = 0) {
+  if (target < 0) {
+    return;
+  } else if (target === 0) {
+    result.push(combination);
+  } else {
+    for (let i = offset; i < candidates.length; i++) {
+      backtrack(result, candidates, target - candidates[i], [...combination, candidates[i]], i);
+    }
+  }
+}