SOURCE

/**
 * @param {number[]} candidates
 * @param {number} target
 * @return {number[][]}
 * 
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]]
 */
var combinationSum3 = function(k, n) {
    var result = [];
    var path = [];
    var startIndex = 1;
    function getSum(total, num) {
        return total + num;
    }
    function backtracking(index){
        var num = path.reduce(getSum,0);
        if(path.length>=k || num>=n){
            if(num===n&&path.length===k){
                result.push([...path]);
            }
            return
        }
        for(var i=index;i<(n-(k-path.length)+1)&&i<=9;i++){
            path.push(i);
            backtracking(i+1);
            path.pop();
        }
    }
    backtracking(startIndex);
    // return result;
    console.log(result);
};
combinationSum3(2,9);