给定二维数组，返回最大的那个子数组

子数组的元素之和

function largestOfFour(arr) {
  let arrsLengths = arr.map(arr=> arr.reduce((a,b)=> a+b, 0))
  let largestNum = Math.max(...arrsLengths)
  let index = arrsLengths.findIndex(num=> num==largestNum)
  return arr[index]
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);