/** * [组合运算] * @param  {array} arr [进行组合运算的数组] * @example [1,3,2] * @param  {number} num [每几个元素为一组] * @return {array}     [返回组合集合] * @example [[1,3],[1,2],[3,2]] */var combo = function(arr, num) {  var result = [];  var range = function(r, _arr) {    if (r.length == num) {      result.push(r)    } else {      let l = r.length;      for (var i = 0, len = _arr.length - num + l; i <= len; i++) {        range(r.concat(_arr[i]), _arr.slice(i + 1))      }    }  }  range([], arr);  return result}