KMP算法

public class KMP{

  public int kmp_pattern(String T,String P){
    if(T == null || P == null || T.length() == 0 || P.length() == 0 || T.length() < P.length())
       return -1;
    int [] next = getNext(P);//获取匹配字符串的next数组
    int i = 0,j = 0;
    //KMP算法
    while(i < T.length() && j < P.length()){
      if(T.charAt(i) == P.charAt(j)){
         i++;
         j++;
      }else{
         if(j == 0){
           i++;
         }else{
           j = next[j];
         }
      }
    }
    if(j == P.length()){
       return i - j;
    }else{
       return -1;
    }
  }
  /**
    *获取next数组
    */
  public int [] getNext(String s){
    if(s == null || s.length() == 0) return null;
    int [] next = new int [s.length()];
    next[0] = -1;
    for(int i = 1;i < s.length();i++){
       for(int j = i-1;j > 0;j--){
          if(s.substring(0,j).equals(s.substring(i-j,i))){
             next[i] = j;
             break;
          }
       }
    }
    return next;
  }

}