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 = lj = rprint 'bg',i,j,kkey = a[(l+r)>>1]while i<=j:while a[i]<key: i += 1while a[j]>key: j -= 1if (i <= j):temp = a[i]a[i]= a[j]a[j] = tempi += 1j -= 1print i,j,kprint aif (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 = 7select(a,0,len(a)-1,k)print a