n个数求第k大

借用快排的思想，每次选取一个参考值，把区间分成两块，左边都小于参考值，右面都大于参考值（中间如果有剩下就是和参考值相同），然后根据k再去选择排左面还是排右面

def select(a,l,r,k):
	i = l
	j = r
	print 'bg',i,j,k
	key = a[(l+r)>>1]
	while i<=j:
		while a[i]<key: i += 1
		while a[j]>key: j -= 1
		if (i <= j):
			temp = a[i]
			a[i]= a[j]
			a[j] = temp
			i += 1
			j -= 1
	print i,j,k
	print a
	if (k <= j and j > l): select(a,l,j,k)
	elif k >= i and i < r: select(a,i,r,k)

a = [12,9,7,6,3,2,0,1,4,5,10,11,8]
k = 7
select(a,0,len(a)-1,k)
print a