子集 #19

var subsets = function(nums) {
    const res = [];
    const backtrack = (path, l, start) => {
        if (path.length === l) {
            res.push(path)
            return;
        }
        for(let i=start; i<nums.length; i++) {
            backtrack(path.concat(nums[i]), l, i + 1)
        }
    }
    for (let i = 0; i <= nums.length; i++) {
        backtrack([], i, 0)
    }
    return res;
}