程序员代码面试指南刷题--第四章.最长公共子串

题目描述:
给定两个字符串str1和str2,输出两个字符串的最长公共子串，如果最长公共子串为空，输出-1。
输入描述:

输入包括两行，第一行代表字符串srr1，第二行代表字符串str2。
输出描述:

输出包括一行，代表最长公共子串。

示例1

输入

1AB2345CD
12345EF

输出

2345

解法一：动态规划

思路： 还是难在确定如何抽象

import java.io.*;
import java.util.*;
public class Main{
     
    public static void main(String[] args) throws Exception{
     
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s1 = br.readLine().trim();
        String s2 = br.readLine().trim();
        String res = findCommon(s1,s2);
        System.out.println(res);
    }
    public static String findCommon(String s1,String s2){
     
        if(s1==null&&s2==null) return "-1";
        if(s1==null||s2==null) return "-1";
        if(s1.length()==0||s2.length()==0) return "-1";
        char[] arr1 = s1.toCharArray();
        char[] arr2 = s2.toCharArray();
        int[][] dp = getdp(arr1,arr2);
        int index = 0;
        int max = 0;
        for(int i=0;i<arr1.length;i++){
     
            for(int j=0;j<arr2.length;j++){
     
                if(dp[i][j]>max){
     
                    max = dp[i][j];
                    index = i;
                }
            }
        }
        if(max==0) return "-1";
        return s1.substring(index-max+1,index+1);
    }
    public static int[][] getdp(char[] arr1,char[] arr2){
     
        int[][] dp = new int[arr1.length][arr2.length];
        for(int i=0;i<arr1.length;i++){
     
            dp[i][0] = arr1[i]==arr2[0]?1:0;
        }
        for(int i=0;i<arr2.length;i++){
     
            dp[0][i] = arr1[0]==arr2[i]?1:0;
        }
        for(int i=1;i<arr1.length;i++){
     
            for(int j=1;j<arr2.length;j++){
     
                dp[i][j] = arr1[i]==arr2[j]?dp[i-1][j-1]+1:0;
            }
        }
        return dp;
    }
}

解法二：优化空间

思路： 因为只用了对角线的上一个

import java.io.*;
import java.util.*;
public class Main{
     
    public static void main(String[] args) throws Exception{
     
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s1 = br.readLine().trim();
        String s2 = br.readLine().trim();
        String res = findCommon(s1,s2);
        System.out.println(res);
    }
    public static String findCommon(String s1,String s2){
     
        if(s1==null&&s2==null) return "-1";
        if(s1==null||s2==null) return "-1";
        if(s1.length()==0||s2.length()==0) return "-1";
        char[] arr1 = s1.toCharArray();
        char[] arr2 = s2.toCharArray();
        //int[][] dp = getdp(arr1,arr2);
        int index = 0;
        int max = 0;
        for(int i=0;i<arr1.length;i++){
     
            int x = i,y = 0;
            int tmp = 0;
            while(x<arr1.length&&y<arr2.length){
     
                if(y==0){
     
                    tmp = arr1[x]==arr2[0]?1:0;
                }else{
     
                    tmp = arr1[x]==arr2[y]?tmp+1:0;                    
                }
                if(tmp>max){
     
                    max = tmp;
                    index = x;
                }
                x++;
                y++;
            }
        }
        for(int i=0;i<arr2.length;i++){
     
            int y = i,x = 0;
            int tmp = 0;
            while(x<arr1.length&&y<arr2.length){
     
                if(x==0){
     
                    tmp = arr2[y]==arr1[0]?1:0;
                }else{
     
                    tmp = arr1[x]==arr2[y]?tmp+1:0;                    
                }
                if(tmp>max){
     
                    max = tmp;
                    index = x;
                }
                x++;
                y++;
            }
        }
        if(max==0) return "-1";
        return s1.substring(index-max+1,index+1);
    }
}