非递归实现归并排序

思想：利用的还是分治思想，只不过这次我们把数组以2的倍数划分，然后进行两两合并，划分代码如下：

public static void merge(int[] arr){
        for(int i = 1;i<arr.length;i *= 2){
            mergeSort(arr,i);
        }
    }

合并代码如下：

public static void mergeSort(int[] arr, int gap) {
        int[] tmp = new int[arr.length];
        int index = 0;
        int start1 = 0;
        int end1 = start1 + gap - 1;
        int start2 = end1 + 1;
        int end2 = (start2 + gap - 1)>=arr.length?arr.length-1:start2+gap-1;
        while(start2<arr.length){
            while(start1<=end1&&start2<=end2){
                if(arr[start1]<arr[start2]){
                    tmp[index++] = arr[start1++];
                }else{
                    tmp[index++] = arr[start2++];
                }
            }
            while(start1<=end1){
                tmp[index++] = arr[start1++];
            }
            while(start2<=end2){
                tmp[index++] = arr[start2++];
            }
            start1 = end2+1;
            end1 = start1 + gap - 1;
            start2 = end1 + 1;
            end2 = (start2 + gap - 1)>=arr.length?arr.length-1:start2+gap-1;
        }
        while(start1<arr.length){
            tmp[index++] = arr[start1++];
        }
        for(int j = 0;j<tmp.length;j++){
            arr[j] = tmp[j];
        }

    }