归并排序

function mergeSort(arr) {                                  //参数类型：数组；返回：数组。
        if (arr.length < 2) {
            return arr;
        }

        const middle = Math.floor(arr.length / 2);         //中间值
        const left = arr.slice(0,middle);                  //left[]
        const right = arr.slice(middle);                   //right[]  
        return merge(mergeSort(left),mergeSort(right));    //递归两次
    }

    function merge(left,right) {
        var newArr = [];
        while (left.length && right.length) {              
            if (left[0] < right[0]) {                       //子序列比较大小
                newArr.push(left.shift());
            } else {
                newArr.push(right.shift());
            }
        }
        return newArr.concat(left,right);                    //返回合并好的了子序列
    }

    alert(mergeSort([54,24,15,95,75,35,68,78,12,31,108]));   //12,15,24,31,35,54,68,75,78,95,108