Implement strStr()

class Solution 
{
public:
  char *strStr(char *haystack, char *needle) 
  {
    const int n = strlen(haystack), m = strlen(needle);        
    if (!m)
      return haystack;
    int next[m], i, j;
    next[i = 0] = j = -1;
    //求取next数组
    while (i < m - 1)
    {
      if (j == -1 || needle[i] == needle[j])
        next[++i] = ++j;
      else
        j = next[j];
    }
    //模式匹配
    for (i = j = 0; i < n;)
    {
      if (j == -1 || haystack[i] == needle[j])
        ++i, ++j;
      else
        j = next[j];
      if (j == m)
        return haystack + i - m;
    }
    return NULL;
  }
};