2.合并二维有序数组成一维有序数组，归并排序的思路

function merge(left, right) {
    let result = []
    while (left.length > 0 && right.length > 0) {
        if (left[0] < right[0]) {
            /*shift()方法用于把数组的第一个元素从其中删除，并返回第一个元素的值。*/
            result.push(left.shift())
        } else {
            result.push(right.shift())
        }
    }
    return result.concat(left).concat(right)
}
function mergeSort(arr) {
    if (arr.length === 1) {
        return arr
    }
    while(arr.length > 1){
        let arrayItem1 = arr.shift();
        let arrayItem2 = arr.shift();
        let mergeArr = merge(arrayItem1, arrayItem2);
        arr.push(mergeArr);
    }
    return arr[0]
}

let arr1 = [[1,2,3],[4,5,6],[7,8,9],[1,2,3],[4,5,6]];
let arr2 = [[1,4,6],[7,8,10],[2,6,9],[3,7,13],[1,5,12]];
console.log(mergeSort(arr1))
console.log(mergeSort(arr2))