计数排序和归并排序

计数排序

计数排序是一种非比较排序。

原理：

代码：

void CountSort(int* nums, int numsSize) {
    int max = nums[0], min = nums[0];
    for (int i = 0; i < numsSize; i++) {
        if (nums[i] > max) {
            max = nums[i];
        }
        if (nums[i] < min) {
            min = nums[i];
        }
    }
    int range = (max - min) + 1;
    int* count = (int*)malloc(sizeof(int) * range);
    memset(count, 0, sizeof(int) * range);
    //统计次数
    for (int i = 0; i < numsSize; i++) {
        count[nums[i] - min]++;
        //count[nums[i] ]++;
    }
    int j = 0;
    for (int i = 0; i < range; i++) {
        while (count[i]--) {
            nums[j++] = i + min;
            //nums[j++] = i;
            //++j;
        }
    }
    free(count);
}

for循环中i的范围应该注意，是range不是numsSize。

归并排序

原理：

代码：

void _MergeSort(int* nums, int left, int right,int*tmp) {
    if (left >= right) {
        return;
    }
    int mid = (left + right) >> 1;
    //[left,mid]  [mid+1,right]有序后，即可进行归并
    _MergeSort(nums, left, mid, tmp);
    _MergeSort(nums, mid+1, right, tmp);
    int begin1 = left, end1 = mid;
    int begin2 = mid + 1, end2 = right;
    int index = left;
    while (begin1 <= end1 && begin2 <= end2) {
        if (nums[begin1] < nums[begin2]) {
            tmp[index++] = nums[begin1++];
        }
        else {
            tmp[index++] = nums[begin2++];
        }
    }
    while (begin1 <= end1) {
        tmp[index++] = nums[begin1++];
    }
    while (begin2 <= end2) {
        tmp[index++] = nums[begin2++];
    }
    for (int i = left; i <= right; i++) {
        nums[i] = tmp[i];
    }
}
void MergeSort(int* nums, int numsSize) {
    int* tmp = (int*)malloc(sizeof(int) * numsSize);
    _MergeSort(nums, 0, numsSize - 1,tmp);
    free(tmp);
}

归并排序核心是利用_MergeSort，然后在_MergeSort中分区间递归。

递归后归并。最小区间例如【0,0】【1,1】选小的往下放，然后拷贝回原数组。