https://leetcode.com/problems/combination-sum/

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/**
 * @param {number[]} candidates
 * @param {number} target
 * @return {number[][]}
 */
const combinationSum = function(candidates, target) {
    candidates.sort()
    let res = []
    const backtrack = function(nums, remain, tmp = [], i = 0) {
        if (remain < 0) {
            return
        }
        if (remain === 0) {
            return res.push([...tmp]) // Notice: [...tmp] is important
        }
        for (let len = nums.length; i < len; i++) {
            tmp.push(nums[i])
            backtrack(nums, remain - nums[i], tmp, i)
            tmp.pop()
        }
    }
    backtrack(candidates, target)
    return res
}