所有数字都是正整数。
解集不能包含重复的组合。 

输入: k = 3, n = 7
输出: [[1,2,4]]

输入: k = 3, n = 9
输出: [[1,2,6], [1,3,5], [2,3,4]]

class Solution {
    protected $ans = [];
    /**
     * @param Integer $k
     * @param Integer $n
     * @return Integer[][]
     */
    function combinationSum3($k, $n){
        $this->backtrack($k, $n, 1, []);
        return $this->ans;
    }

    function backtrack($k, $target, $start, $path){
        if ($target == 0 && count($path) == $k) {
            $this->ans[] = $path;
            return;
        }
        for ($i = $start; $i <= 9; ++$i) {
            if ($i > $target) continue;
            if (count($path) >= $k) continue;
            $path[] = $i;
            $this->backtrack($k, $target - $i, $i + 1, $path);
            array_pop($path);
        }
    }
}