O(n^2)的时间复杂度

使用优化算法减小时间复杂度

示例代码：

1. Bubble Sort

void bubbleSort(int arr[], int n) { int i, j; for (i = 0; i < n-1; i++)

for (j = 0; j < n-i-1; j++)
    if (arr[j] > arr[j+1])
        swap(&arr[j], &arr[j+1]);

}

时间复杂度为O(n^2)

2. Selection Sort

void selectionSort(int arr[], int n) { int i, j, min_idx;

for (i = 0; i < n-1; i++)
{
    min_idx = i;
    for (j = i+1; j < n; j++)
      if (arr[j] < arr[min_idx])
        min_idx = j;

    swap(&arr[min_idx], &arr[i]);
}

}

时间复杂度为O(n^2)

优化算法：

1. Merge Sort

void merge(int arr[], int l, int m, int r) { int i, j, k; int n1 = m - l + 1; int n2 = r - m;

/* create temp arrays */
int L[n1], R[n2];

/* Copy data to temp arrays L[] and R[] */
for (i = 0; i < n1; i++)
    L[i] = arr[l + i];
for (j = 0; j < n2; j++)
    R[j] = arr[m + 1+ j];

/* Merge the temp arrays back into arr[l..r]*/
i = 0; // Initial index of first subarray
j = 0; // Initial index of second subarray
k = l; // Initial index of merged subarray
while (i < n1 && j < n2)
{
    if (L[i] <= R[j])
    {
        arr[k] = L[i];
        i++;
    }
    else
    {
        arr[k] = R[j];
        j++;
    }
    k++;
}

/* Copy the remaining elements of L[], if there
   are any */
while (i < n1)
{
    arr[k] = L[i];
    i++;
    k++;
}

/* Copy the remaining elements of R[], if there
   are any */
while (j < n2)
{
    arr[k] = R[j];
    j++;
    k++;
}

}

/* left is for left index and right is right index of the