第K个数
其中,快速选择排序的
if l >= r: return
可以写为if l == r: return
思路
n, k = list(map(int,input().split()))
alist = list(map(int,input().split()))
def kSort(q, l, r, k):
if l >= r: return q[l]
i = l - 1
j = r + 1
x = q[i + j >> 1]
while i < j:
while True:
i += 1
if not q[i] < x: break
while True:
j -= 1
if not q[j] > x: break
if i < j: q[i], q[j] = q[j], q[i]
length = j - l + 1
if k <= length:
return kSort(q, l, j, k)
else:
return kSort(q, j + 1, r, k - length)
print(kSort(alist, 0, n - 1, k))