歸併排序

利用遞迴實現分治。每次排序時間複雜度是O（N），一共需要 LogN 次。

#include <stdlib.h>
#include <time.h>

#define size 100

static int n[size];
static int t[size];


void msort(int n[], int start, int end)
{
        int restart = start;
        int left = start;
        int leftend = (start + end) / 2;
        int
 
 right = leftend + 1;
        int rightend = end;

        if(left != leftend)
                msort(n, left, leftend);
        if(right != rightend)
                msort(n, right, rightend);
        int i = left;
        while(left <= leftend && right <= rightend) {
                if
 
(n[left] < n[right])
                        t[i++] = n[left++];
                else
                        t[i++] = n[right++];
        }
        while(left <= leftend)
                t[i++] = n[left++];
        while(right <= rightend)
                t[i++] = n[right++];
        while
 
(restart <= end) {
                n[restart] = t[restart];
                restart++;
        }
}

int main()
{
        time_t seed;

        seed = time(0);
        srandom(seed);

        for(int i = 0; i < size; i++) {
                n[i] = random()%1000;
        }
        msort(n, 0, size - 1);
        for(int i = 0; i < size; i++)
                printf("%d ",n[i]);
        printf("\n");
        return 0;
}