题目

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.

不重复的，且元素都为正的候选数组中，找出所有可能的元素组合加起来的值等于目标值

• Example1

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

• Example2

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

• 利用递归解决（类似于找硬币问题）

var combinationSum = function(arr, target) {
    let res = []
    let temp = []
    function find(target,temp,index){
        for(let i = index; i >= 0; i--){
            if(arr[i] > target){
                continue // 继续循环，不执行下面的值
            }else if(arr[i] === target){
                res.push(temp.concat(arr[i]))
            }else{
                //保存当前的i值，下次调用开始的值还是使用arr[i]累加
                //[1,2,3] target=9 ，可以3+3+3
                find(target-arr[i], temp.concat(arr[i]),i)
            }
        }

    }
    // 递归调用
    find(target,temp,arr.length-1)
    return res
};