Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4],]
这个几乎是最标准的回溯问题
筛选组合的条件有两个，一个是组合的长度，一个是不选重复元素

var combine = function(n, k) {
    var result = [];
    if (n===0||k===0)
        return result;
    var help = function(res,start){
        if (res.length===k)
            result.push(res.concat());
        else {
            for(var i = start;i <= n; i++) {
                res.push(i);
                help(res,i+1);
                res.pop();
            }
        }   
    };
    help([],1);
    return result;
};