28.LeetCode Implement strStr()(meidum)[字符串 子串匹配]

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

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strStr(str1,str2)的功能是判断str2是否是str1的子串，解题思路如下：首先判断str2是否是空串，空串是任何字符串的子串返回0；当str2的长度大于str1的时候，那么str2肯定不是str1的子串，返回-1；str2从开始字串针对str1开始扫描，但是由于后面的子串需要完全匹配，所以对str1的扫描只需要扫描到<=str1.size()-str2.size()的地方为止。

class Solution {
public:
int strStr(string haystack, string needle) {
if(needle.size()>haystack.size()) return -1;
if(needle.size() == 0) return 0;
int pos = -1;
int i = 0;
for(int j=0;j<=haystack.size()-needle.size();j++)
{
if(haystack[j] == needle[i])
{
int p = i,q = j;
while(p<needle.size()&&q<haystack.size())
{
if(needle[p]!= haystack[q])
break;
else
{
++p;++q;
}
}
//cout<<p<<endl;
if(p == needle.size())
{
pos = j;
break;
}
}
}
return pos;
}
};