三种常见的排序算法

void insertion_sort(int *A, int length) {
    int i, key;
    for (int j = 1; j < length; j++) {//从第二个开始判断，保证前面有序
        key = A[j];
        i = j - 1;
        while (i >= 0 && A[i] > key) {
            A[i + 1] = A[i];
            i--;
        }
        A[i + 1] = key;//插入
    }
}

void merge(int *A, int p, int q, int r) {//q属于[q, r]
    //分治保证L, R数组是有序的
    int n1, n2, *L, *R, i, j;
    n1 = q - p + 1;//[p, q]
    n2 = r - q;//(q, r]
    L = new int[n1];
    R = new int[n2];
    for (i = 0; i < n1; i++)
        L[i] = A[p + i];
    for (j = 0; j < n2; j++)
        R[j] = A[q + 1 + j];
    i = j = 0;
    for (int k = p; k <= r; k++) {
        if (j == n2)//任意一边选完了，直接进去
            A[k] = L[i++];
        else if (i == n1)
            A[k] = R[j++];
        else {
            if (L[i] < R[j])
                A[k] = L[i++];
            else
                A[k] = R[j++];
        }
    }
    delete[] L;
    delete[] R;
}

void merge_sort(int *A, int p, int r) {//p=0, r=n-1
    if (p < r) {
        //递归实现分治，和二叉树一样，p<r自动实现过滤没有用的节点
        merge_sort(A, p, (p + r) / 2);
        merge_sort(A, (p + r) / 2 + 1, r);
        merge(A, p, (p + r) / 2, r);//此函数实现合并
    }
}

void bubble_sort(int *A, int length) {
    int mid;
    for (int i = 0; i < length; i++) {
        for (int j = length - 1; j > i; j--)
            if (A[j] < A[j - 1]) {//双双相比
                mid = A[j];
                A[j] = A[j - 1];
                A[j - 1] = mid;
            }
    }
}