最长公共子串

public int[][] getdp(char[] str1, char[] str2){
    int[][] dp = nex int[str1.length][str2.length];
    for (int i =0; i < str1.length; i++){
       if (str1[i] == str2[0]){
            dp[i][0] = 1;
        }
    }
    for (int j =0; j < str2.length; j++){
       if (str1[0] == str2[j]){
            dp[0][j] = 1;
        }
    }
    for (int i = 1; i < str1.length; i++){
      for (int j = 1; j < str2.length; j++){
          if(str1[i] == str2[j]){
             dp[i][j] = dp[i-1][j-1] +1;
          }
        }
    }
    return dp;
}

public String (String str1, String str2){
  if (str1 == null || str2 == null || str1 == "" || str2 == ""){
        return "";
  }
    char[] chs1 = str1.toCharArray();
   char[] chs2 = str2.tocharArray();
   
 int[][] dp = getdp(chs1, chs2);
 
 int end = 0;
    int max = 0;
    
 for (int i = 0; i< chs1.length; i++){
       for (int j =0; j < chs2.length; j++){
           if (dp[i][j] > max){
             end = i;
             max = dp[i][j];
          }
            
     }
    }
    return str1.substring(end-max+1, end+1);
    
}

public String lcat2(String str1, String str2){
  if (str1 == null || str2 == null || str1 == "" || str2 == ""){
        return "";
  }
    char[] chs1 = str1.toCharArray();
   char[] chs2 = str2.toCharArray();
   
 int row = 0;
    int col = chs2.length -1;
   int max = 0;
    int end = 0;
    
 while(row < chs1.length){
        int i = row;
        int j = col;
        int len = 0;
        while (i < chs1.length && j < chs2.length){
           if (chs1[i] != chs2[j]){
                len = 0;
         }else{
              len++;
           }
            
         
         if (len > max) {
             end = i;
             max = len;
           }
            
         i++;
         j++;
     }
        if (col > 0){
            col--;
       }
        else{
           row++;
       }
    }
    return str1.substring(end -max +1,end +1);
}