28. Implement strStr（python+cpp）

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题目：

Implement strStr().
Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Example 1:

Input: haystack = "hello", needle = "ll" 
Output: 2 

Example 2:

Input: haystack = "aaaaa", needle = "bba" 
Output: -1 

Clarification:
What should we return when needleis an empty string? This is a great question to ask during an interview.
For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C’s strstr() and Java’s indexOf().

解释：
作弊的解法，内置库函数，注意不要用index()，因为如果不存在的话，index()会报错而find()不会报错而是返回-1。
python代码：

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        return haystack.find(needle) 

不作弊的写法，需要用在切片（子字符串）
python代码：

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if not needle:
            return 0;
        
        n=len(haystack)
        m=len(needle)
        for  i in range(n-m+1):
            if haystack[i:m+i]==needle:
                return  i
        return -1    

c++代码：

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle.size()==0)
            return 0;
        int m=needle.size();
        int n=haystack.size();
        for(int i=0;i<n-m+1;i++)
        {
            if (haystack.substr(i,m)==needle)
                return i;
        }
        return -1;
    }
};

总结：
使用子字符串，每次在haystack中取和needle长度一样的子字符串和needle比较即可。