KMP之Z-function (扩展kmp)

http://codeforces.com/blog/entry/3107

// s[0, ..., n-1], z[0] = 0
// z[i] is the length of the longest substring starting from s[i] which is also a prefix of s
// O(n), R increases...
int L = 0, R = 0;
for (int i = 1; i < n; i++) {
  if (i > R) {
    L = R = i;
    while (R < n && s[R-L] == s[R]) R++;
    z[i] = R-L; R--;
  } else {
    int k = i-L;
    if (z[k] < R-i+1) z[i] = z[k];
    else {
      L = i;
      while (R < n && s[R-L] == s[R]) R++;
      z[i] = R-L; R--;
    }
  }
}