# 子串第一次出现位置

利用for else。KMP？

```python
class Solution:
    def strStr(self, source, target):
        if source is None or target is None:
            return -1

        for i in range(len(source) - len(target) + 1):  # 从到第一个位置，到倒数第子串长位置遍历
            for j in range(len(target)):  # 遍历一遍子串
                if source[i + j] != target[j]:  # 只要i开头的串，有一个和子串不一样，就退出开始下一个起点
                    break
            else:  # python 魔法，当上面的for全部正常运行完时，执行else，也就是全部匹配子串不break
                return i
        return -1


s = Solution()
a = s.strStr('acdbcdbdse', 'dbd')
print(a)


#5
```


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