自學記錄

public class MergeSort {
    public static void main(String[] args){
        int[] b={4,4,1,2,-9,10,101,2,23,9,-40,13,14,5,-8,2,1};
        mergeSort(b);
        SortTest.show(b);
    }
    public static void mergeSort(int[] a){
        int low,mid,high,step=1,len=a.length;
        //step不能大於等於區間長度,只能小於;
        while (step<len){
            low=0;//從頭開始找
            // low +step-1+1<=length-1,保證high存在,若連長度為到mid為1的high值都越界了,就不執行迴圈,例如 (0,1),(2,3),4
            while (low+step<len){
                mid=low+step-1;
                high=mid+step;//mid+1+step-1
                if(high>len-1){ //high別越界了
                    high=len-1;
                }
                merge(a,low,mid,high);
                low=high+1;
            }
            step*=2;//一步兩,兩步四個...
        }

    }
    public static void merge(int[] arr, int low, int mid ,int high){
        int i=low,j=mid+1,k=0;
        int[] temp = new int[high-low+1];//臨時矩陣;
        while (i<=mid && j<=high){
            if(arr[i]<=arr[j]){         //比較大小;
                temp[k++]=arr[i++];
            }
            else{
                temp[k++]=arr[j++];
            }
        }
        while(i<=mid){
            temp[k++]=arr[i++];//把剩下的補齊
        }
        while (j<=high){
            temp[k++]=arr[j++];
        }
        for(i=low,k=0;i<=high;i++,k++){
            arr[i]=temp[k];//寫回去;
        }
    }

}