Combination Sum.js

// Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

// The same repeated number may be chosen from candidates unlimited number of times.

// Note:

// All numbers (including target) will be positive integers.
// The solution set must not contain duplicate combinations.
// Example 1:

// Input: candidates = [2,3,6,7], target = 7,
// A solution set is:
// [
//   [7],
//   [2,2,3]
// ]
// Example 2:

// Input: candidates = [2,3,5], target = 8,
// A solution set is:
// [
//   [2,2,2,2],
//   [2,3,3],
//   [3,5]
// ]

var combinationSum = function(candidates, target) {
    let result = [];
    
    function backtrack(current, target, start) {
        if (target < 0) return;
        if (target === 0) {
            result.push(current);
            return;
        }
        for (let i = start; i < candidates.length; i++) {
					// since we can reuse numbers, we can reuse i for next iteration
            backtrack([...current, candidates[i]], target - candidates[i], i);
        }
    }
    
    backtrack([], target, 0);
    return result;
};