获取最大流的Ford-Fulkerson算法中的空指针异常

Question

我已经写了一个代码，用于从图中获得最大流量，该图被硬编码到main中。我按照说明操作，但我一直收到这个错误，而且我不确定为什么。我试着更改了很多部分并进行了调试，但我还是搞不清楚。这是错误：

Exception in thread "main" java.lang.NullPointerException
    at lib280.graph.FF.dft(FF.java:78)
    at lib280.graph.FF.isRes(FF.java:67)
    at lib280.graph.FF.findMax(FF.java:40)
    at lib280.graph.FF.main(FF.java:117)

下面是我的代码：

package lib280.graph;

public class FF {
    public boolean[] visited; 
    double parent[]; 
    public int maxflow; 

    public int findMax(double G[][], int s, int t){
        maxflow = 0;
        int a = G.length; //length of rows 
        int b = G[0].length; // length of columns
        int v = a*b; // vertices
        int i, j;
        double c;
        double[][] residual = new double[a][b]; //the initializion for the residuals 
        //Create residual of G 
        for (i=0;i<a;i++){
            for (j=0; j<b;j++){
                residual[i][j] = G[i][j];
            }
        }
        //while the path from s to t is residual, find max flow 
        while (isRes(residual, s, t, parent)){
            int path = Integer.MAX_VALUE; //(LINE 40)
            for (double h=t; h<s; h=parent[(int)h]){
                c = parent[(int)h];
                path = Math.min(path, (int)residual[(int)c][(int)h]);
            }

            for (double h=t; h<s; h=parent[(int)h]){
                c = parent[(int)h];
                residual[(int)c][(int)h] -= path; //residual for the edge
                residual[(int)h][(int)c] += path; //residual for the reverse edge
            }

            maxflow = maxflow+path; //update max
        }
        return maxflow;
    }

    public boolean isRes(double G[][], int s, int t, double parent[]){
        dft(G, s);
        return (visited[t]==true); //(Line 67)
        }

    public void dft(double G[][], int s){
        for (int i=0;i<G.length;i++){ // initializing every vertex not visited 
            visited[i] = false; // (Line 78)
        }
        dftHelper(G, s);
    }

    public void dftHelper(double G[][], int s){
        visited[s] = true;

        for (int i=0; i<G.length; i++){
            if (visited[i]==false && G[s][i]>0){ // for each node adjacent to s and if it is not reached
                dftHelper(G, i);
            }
        }
    }

这是main的精简版本：

public static void main(String args[]) throws Exception {
        FF a = new FF();
        double G[][] = hardcoded graph

        int x = a.findMax(G, 0, 5);
        System.out.println("Max flow" + x); //(Line 117)
        System.out.println("Final Residual Capacities: ");
    }
}

Answer 1

看起来你在FF中声明了'visited‘，但是从来没有构造过一个实际的数组。