python小练

素数筛：

isprime = []
ans = []

for i in range(0, 101):
    isprime.append(True)
isprime[0] =  False
isprime[1] = False
for i in range(2, 11):
    if isprime[i] == True:
        t = int(100 / i) + 1
        for j in range(2, t):
            isprime[i*j] = False

for i in range(1, 101):
    if isprime[i] == True:
        ans.append(i)
# for i in ans:
#     print i,
# print

 最大公因数：

def gcd(a, b):
    if b == 0:
        return a
    else:
        return gcd(b, a % b)

 二进制下a有几个1：

def cnt(a):
    cnt = 0
    while a:
        a = a & (a - 1)
        cnt += 1
    return cnt

print(cnt(a))

 KMP算法：

a = "abcdabc"
b = "abc"
na = len(a)
nb = len(b)
pre = [0] * 100
pre[0] = -1
j = 0
k = -1
while(j < nb):
    if k == -1 or b[j] == b[k] :
        j += 1
        k += 1
        pre[j] = k
    else:
        k = pre[k]
ans = 0
i = 0
j = 0
while(i < na):
    if a[i] == b[j] or j == -1:
        i += 1
        j += 1
    else:
        j = pre[j]
    if j == nb:
        j = 0   #res.append(j)
        ans = 1
if ans == 1:
    print("YES")
else:
    print("NO")