# KMP字符串

```python
def KMP_algorithm(string, substring):
    '''
    KMP字符串匹配的主函数
    若存在字串返回字串在字符串中开始的位置下标，或者返回-1
    '''
    pnext = gen_pnext(substring)
    n = len(string)
    m = len(substring)
    i, j = 0, 0
    while (i<n) and (j<m):
        if (string[i]==substring[j]):
            i += 1
            j += 1
        elif (j!=0):
            j = pnext[j-1]
        else:
            i += 1
    if (j == m):
        return i-j
    else:
        return -1


def gen_pnext(substring):
    """
    构造临时数组pnext
    """
    index, m = 0, len(substring)
    pnext = [0]*m
    i = 1
    while i < m:
        if (substring[i] == substring[index]):
            pnext[i] = index + 1
            index += 1
            i += 1
        elif (index!=0):
            index = pnext[index-1]
        else:
            pnext[i] = 0
            i += 1
    return pnext

if __name__ == "__main__":
    string = 'abcxabcdabcdabcy'
    substring = 'abcdabcy'
    out = KMP_algorithm(string, substring)
    print(out) #8
```


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