将文件读入二维数组-获取ArrayIndexOutOfBoundsException（）

Question

Hi I've been doing Project Euler problems lately but I'm having trouble with Problem 18, here it is:

By starting at the top of the triangle below and moving to adjacent numbers on the row below, the maximum total from top to bottom is 23.

3

7 4

2 4 6

8 5 9 3

That is, 3 + 7 + 4 + 9 = 23.

Find the maximum total from top to bottom of the triangle below:

75

95 64

17 47 82

18 35 87 10

20 04 82 47 65

19 01 23 75 03 34

88 02 77 73 07 63 67

99 65 04 28 06 16 70 92

41 41 26 56 83 40 80 70 33

41 48 72 33 47 32 37 16 94 29

53 71 44 65 25 43 91 52 97 51 14

70 11 33 28 77 73 17 78 39 68 17 57

91 71 52 38 17 14 91 43 58 50 27 29 48

63 66 04 68 89 53 67 30 73 16 69 87 40 31

04 62 98 27 23 09 70 98 73 93 38 53 60 04 23

NOTE: As there are only 16384 routes, it is possible to solve this problem by trying every route. However, Problem 67, is the same challenge with a triangle containing one-hundred rows; it cannot be solved by brute force, and requires a clever method! ;o)

My algorithm for finding the max total is correct. I tested it by manually entering the numbers in a 2d array and the program ran fine. The reason I don't want to do it that way is as this Problem says Problem 67 is the same except much larger numbers and I don't want to be typing numbers all day and also I want to practice manipulating files etc.

Anyway to the point I suppose the best thing I can do really is show you my code and the errors I'm getting. I did some debugging and it seems to be how I'm converting the string of numbers into an array of digits. When I run the program it gives an ArrayIndexOutOfBoundsException(). The .txt file consists of the large group of numbers in the problem description above.

public static void main(String[] args) throws Exception
{
    Problem18 p18 = new Problem18() ;
    p18.maxTotalPath() ;
}

public int[][] readFile() throws Exception
{
    ClassLoader loader = Thread.currentThread().getContextClassLoader() ;
    InputStream file = loader.getResourceAsStream("Triangle.txt") ;

    Scanner scan = new Scanner(file) ;

    int[][] triangle = new int[15][15] ;
    int m = 0, n = 0 ;
    String line ;
    while(scan.hasNext())
    {
        line = scan.nextLine() ;
        String[] numbers = line.split(" ") ;

        for(int i = 0 ; i < numbers.length ; i++)
        {
            triangle[m][n] = Integer.parseInt(numbers[i]) ;
            //System.out.print(triangle[m][n] + " ") ;
            n += 1 ;
        }
        n = 0 ;
        // System.out.println("") ;
        m += 1 ;
    }
    scan.close() ;
    return triangle ;
}

public void maxTotalPath() throws Exception
{
    int[][] arr = readFile() ;
    for (int i = arr.length - 2 ; i >= 0 ; i--) 
    {
        for (int j = 0 ; j < arr[i].length; j++) 
        {
            arr[i][j] += Math.max(arr[i + 1][j], arr[i + 1][j + 1]); 
        }
    }
    System.out.println(Integer.toString(arr[0][0])) ;
}

The errors then are:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 15
at com.jconnolly.projeuler.problems.Problem18.maxTotalPath(Problem18.java:83)
at com.jconnolly.projeuler.problems.Problem18.main(Problem18.java:44)

Any help would be much appreciated thanks!

Answer 1

maxTotalPath内部循环应转到arr [i] .length-1，这将至少停止异常

Answer 2

Well, your readFile function is working as it should. Your maxTotalPath is trying to access the wrong memory in the inner loop. If j goes all the way to arr[i].length-1 , then when you access arr[i+1][j+1] , you will be reaching memory beyond the bounds of the array.

for (int j = 0 ; j < arr[i].length-1; j++) 
{
    arr[i][j] += Math.max(arr[i + 1][j], arr[i + 1][j + 1]); 
}

Correcting the wrong answer

I think this is what you need to do

for (int j = 0 ; j < arr[i].length; j++) 
{
    if (j != arr[i].length-1) {
        arr[i][j] += Math.max(arr[i + 1][j], arr[i + 1][j + 1]); 
    } else {
        arr[i][j] += arr[i+1][j];
    }
}