数组 有序合并

function ArrayExchange(arr, num) {
                var r = [];
                (function f(t, a, n) {
                    if (n == 0) return r.push(t);
                    for (var i = 0, l = a.length; i <= l - n; i++) {
                        f(t.concat(a[i]), a.slice(i + 1), n - 1);
                    }
                })([], arr, num);
                return r;
            }

ArrayExchange([1,2,3],2)