动态规划之 longest common substring最大公共子串

这题是经典的动态规划算法，有些帖子没说清楚，然后看了下wikipedia，基本上一张图能看懂递推关系，

基于这张图的直觉，自己写了一版：

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; imaxLength){
					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));
	}

}