快速排序
QUICKSORT(A, p, r)
if p < r
q = PARTITION(A,p,r)
QUICKSORT(A, p, q-1)
QUICKSORT(A, q+1, r)
PARTITION(A, p, r)
x = A[r]
i = p-1
for j = p to r-1
if A[j] <= r
i = i + 1
exchange A[i] with A[j]
exchange A[i+1] with A[r]
return i+ 1
随着程序的执行，数组被划分为4个（可能有空的）区域，for循环没一轮迭代的开始，每一个区域都满足一定
的性质，我们将这些性质作为循环不变量
1，若 p <= k <= i , 则 A[k] <= x
2, 若 i+1 <= k <= j-1, 则 A[k] > x
3, 若 k = r , 则 A[k] = x
