SOURCE

const a = [4,5,135,199,899,999];
const b = [8,34,44,56,231];

// 合并两个数组
function concatArray(arr1, arr2) {
  if(arr1.length < 1) {
    return arr2;
  }else {
    const firstEle = arr1.shift();
    arr2 = insertNumToArray(firstEle, arr2, 0);
    return concatArray(arr1, arr2)
  }
}

// 将数字插入有序数组
function insertNumToArray(num, arr, index) {
  if(num < arr[index]) {
    arr.splice(index, 0, num);
    return arr;
  }else if(index == arr.length) {
    arr.push(num);
    return arr;
  }else {
    ++index;
    return insertNumToArray(num, arr, index);
  }
}

console.log(concatArray(a, b));