动态规划之 longest common substring最大公共子串

import java.util.Arrays;

public class CommonSubstring {
public static String commonSub(String st1, String st2){
int index1 = 0;
int index2 = 0;
int maxLength = -1;

if(st1==null || st2 == null){
return null;
}
int[][] cache = new int[st1.length()][st2.length()];

for(int[] row: cache){
Arrays.fill(row, -1);
}

//initialize boarder
for(int i=0; i<st2.length();i++){
if(st1.charAt(0)==st2.charAt(i)){
cache[0][i] = (st1.charAt(0)==st2.charAt(i)?1:0);
}
}

for(int i=1; i<st1.length();i++){
if(st2.charAt(0)==st1.charAt(i)){
cache[i][0] = (st2.charAt(0)==st1.charAt(i)?1:0);
}
}

for(int i = 0 ; i < st1.length(); i++){
for(int j = 0 ; j <  st2.length(); j++){
int length = calcLength(st1, st2, i, j, cache);
if(length>maxLength){
maxLength = length;
index1 = i;
index2 = j;
}
}
}
return st1.substring(index1-maxLength+1, index1+1);
}

public static int calcLength(String st1, String st2, int i, int j, int[][] cache){
if(cache[i][j]!=-1){
return cache[i][j];
}else{
return cache[i][j]=(st1.charAt(i)==st2.charAt(j)?calcLength(st1, st2, i-1, j-1, cache)+1:0);
}
}

public static void main(String[] str){
String str1 = "abdwhatthefuckdsgsd";
String str2 = "634whatthefuck8gsg";
System.out.println(commonSub(str1, str2));
}

}