Question

我正在尝试开发一个Java程序来解决https://projecteuler.net/problem=18。但是我遇到了困难，我不确定为什么这段代码不起作用：

int[][] testTriangle = {
            {3},
            {7, 4},
            {2, 4, 6},
            {8, 5, 9, 3}
    };

    HashMap<Integer, Integer> hasGoneDownRoute = new HashMap<Integer, Integer>(); //Stores numbers and start positions to numbers

    int largestValue = 0;
    int value = 0;
    int row = testTriangle.length-1;
    for (int startPosition = 0; startPosition <= testTriangle[row].length-1; startPosition++) { //starts from all possible start positions on the bottom row
        for (int y = 1; y<=2; y++) { //executes from the same start position twice to get all possible routes (ACTUALLY THIS MIGHT BE WRONG!?)
            while (row != 0) { //until it reaches the top row
                if (startPosition == 0) { //if it's on the far left
                    value += testTriangle[row-1][0];
                }
                else if (startPosition == testTriangle[row].length-1) { //if at's on the far right
                    value += testTriangle[row-1][testTriangle[row-1].length-1]; //set the value to the row above it on the far right
                }
                else { //This never gets called?
                    int noToChooseFrom1 = testTriangle[row-1][startPosition]; //above it and to the right
                    int noToChooseFrom2 = testTriangle[row-1][startPosition-1]; //above it and to the left
                    if (hasGoneDownRoute.containsKey(noToChooseFrom1) && hasGoneDownRoute.get(noToChooseFrom1) == startPosition) { //checks if it has gone down a certain route before
                        value += noToChooseFrom2;
                        hasGoneDownRoute.put(testTriangle[row-1][startPosition-1], startPosition);
                    }
                    else {
                        value += noToChooseFrom1;
                        hasGoneDownRoute.put(noToChooseFrom1, startPosition);
                    }
                }
                row--;
            }
        }
        if (value > largestValue) {
            largestValue = value;
        }
        System.out.println(largestValue);
    }

我刚添加了笔记，试图解释我的思维过程

Answer 1

编辑：看看这个问题我现在的编程技巧要好得多。这可能是一个死的问题但是。我想给出正确答案。

你真正需要的是递归。它允许你以for循环不会的方式向前看。（很容易反正）

创建一个功能

public int findLargestPath(int[][] triangle, row , col) {

    // Check to see if the row and column are actually in the triangle
    if(col == -1 || col >= triangle[row].length) 
    {
        return -1; //If they are not. ensure that this path wont be taken. 
    }
    if(row == triangle.length -1)
    {
        return triangle[row][col]; //Return the value.
    }
    else
    {
        //return this value plus the maximum path below
        return triangle[row][col]+Math.max(findLargestPath(row+1,col),
                                           findLargestPath(row+1,col+1));
    }
}

然后在主内部它将是一个简单的

int[][] testTriangle = {
        {3},
        {7, 4},
        {2, 4, 6},
        {8, 5, 9, 3}
};

System.out.println("The Maximum Path = "+findLargestPath(testTriangle,0,0));

递归是一种跟踪所有路径而无需创建新变量的方法。我没有在我面前有一个IDE。因此可能存在一些错误。如果你想让我解决它。让我知道，什么时候回到IDE会对这一切进行测试。

Answer 2

https://projecteuler.net/problem=18

动态编程方法：

导入java.io。*;

公共类主要{

public static void main(String[] args) {
    int tri[][] = {
        {
            6,
            0,
            0
        },
        {
            2,
            -3,
            0
        },
        {
            9,
            20,
            -1
        }
    };
    System.out.println(maxPathSum(tri)); 
}


static int maxPathSum(int tri[][]) {  
    for (int i = tri.length - 1; i > 0; i--) {
        for (int j = 0; j < i; j++) {
            //second condition in conditional ensures that we don't add 
            //negative numbers, which would only decrease our max sum
            if (tri[i][j] > tri[i][j + 1] && tri[i][j] + tri[i - 1][j] > tri[i - 1][j]) {
                tri[i - 1][j] += tri[i][j];
           } else if (tri[i][j] < tri[i][j + 1] && [i][j + 1] + tri[i - 1][j] > tri[i - 1][j]) {
                tri[i - 1][j] += tri[i][j + 1];
             }
        }
    }
    return tri[0][0];
}

最大路径总和

2 个答案: