Implement strStr()

Description:

Implement strStr().

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

Code1:

vector<int> getNext ( string&t  )
{
size_t n = t.length();
vector<int>next(n,-1);

int i = 0, j = -1;
while ( i < n-1 )
{
if ( j == -1 || t[i] == t[j])
{
++i;
++j;
next[i] = j;
}
else
j = next[j];
}return next;
}
int indexKMP( string& t, string&w, vector<int>& next )
{
int i = 0, j = 0;
int lengthT = t.length();
int lengthW = w.length();

while ( i < lengthT && j < lengthW )
{
if ( j == -1 || t[i] == w[j] )         //继续比较后继字符
{
++i;
++j;
}
else
j = next[j];
}
if ( j == lengthW )
return i - lengthW;
else
return -1;
}
int strStr(string haystack, string needle) {
if (needle == "")
return 0;
if (haystack == "")
return -1;
vector<int>next;
next = getNext(needle);
return indexKMP(haystack,needle,next);
return -1;
}

Code2:

int strStr(string a, string b)
{//a为目标串，b为字串
if (b == "")
return 0;
if (b.size() > a.size() )
return -1;
for (int i = 0; i <= a.size()-b.size(); ++i)
{
int j = 0;
for (; j < b.size(); ++j)
{
if (a[i+j]!=b[j])
break;
}
if (j==b.size())
return i;
}
return -1;
}