Task: Resistor mesh
C: #include <stdio.h>
#include <stdlib.h>
 
#define S 10
typedef struct { double v; int fixed; } node;
 
#define each(i, x) for(i = 0; i < x; i++)
node **alloc2(int w, int h)
{
	int i;
	node **a = calloc(1, sizeof(node*)*h + sizeof(node)*w*h);
	each(i, h) a[i] = i ? a[i-1] + w : (node*)(a + h);
	return a;
}
 
void set_boundary(node **m)
{
	m[1][1].fixed =  1; m[1][1].v =  1;
	m[6][7].fixed = -1; m[6][7].v = -1;
}
 
double calc_diff(node **m, node **d, int w, int h)
{
	int i, j, n;
	double v, total = 0;
	each(i, h) each(j, w) {
		v = 0; n = 0;
		if (i) v += m[i-1][j].v, n++;
		if (j) v += m[i][j-1].v, n++;
		if (i+1 < h) v += m[i+1][j].v, n++;
		if (j+1 < w) v += m[i][j+1].v, n++;
 
		d[i][j].v = v = m[i][j].v - v / n;
		if (!m[i][j].fixed) total += v * v;
	}
	return total;
}
 
double iter(node **m, int w, int h)
{
	node **d = alloc2(w, h);
	int i, j;
	double diff = 1e10;
	double cur[] = {0, 0, 0};

	while (diff > 1e-24) {
		set_boundary(m);
		diff = calc_diff(m, d, w, h);
		each(i,h) each(j, w) m[i][j].v -= d[i][j].v;
	}
 
	each(i, h) each(j, w)
		cur[ m[i][j].fixed + 1 ] += d[i][j].v *
				(!!i + !!j + (i < h-1) + (j < w -1));
 
	free(d);
	return (cur[2] - cur[0])/2;
}
 
int main()
{
	node **mesh = alloc2(S, S);
	printf("R = %g\n", 2 / iter(mesh, S, S));
	return 0;
}

Java: import java.util.ArrayList;
import java.util.List;

public class ResistorMesh {
    private static final int S = 10;

    private static class Node {
        double v;
        int fixed;

        Node(double v, int fixed) {
            this.v = v;
            this.fixed = fixed;
        }
    }

    private static void setBoundary(List<List<Node>> m) {
        m.get(1).get(1).v = 1.0;
        m.get(1).get(1).fixed = 1;

        m.get(6).get(7).v = -1.0;
        m.get(6).get(7).fixed = -1;
    }

    private static double calcDiff(List<List<Node>> m, List<List<Node>> d, int w, int h) {
        double total = 0.0;
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                double v = 0.0;
                int n = 0;
                if (i > 0) {
                    v += m.get(i - 1).get(j).v;
                    n++;
                }
                if (j > 0) {
                    v += m.get(i).get(j - 1).v;
                    n++;
                }
                if (i + 1 < h) {
                    v += m.get(i + 1).get(j).v;
                    n++;
                }
                if (j + 1 < w) {
                    v += m.get(i).get(j + 1).v;
                    n++;
                }
                v = m.get(i).get(j).v - v / n;
                d.get(i).get(j).v = v;
                if (m.get(i).get(j).fixed == 0) {
                    total += v * v;
                }
            }
        }
        return total;
    }

    private static double iter(List<List<Node>> m, int w, int h) {
        List<List<Node>> d = new ArrayList<>(h);
        for (int i = 0; i < h; ++i) {
            List<Node> t = new ArrayList<>(w);
            for (int j = 0; j < w; ++j) {
                t.add(new Node(0.0, 0));
            }
            d.add(t);
        }

        double[] cur = new double[3];
        double diff = 1e10;

        while (diff > 1e-24) {
            setBoundary(m);
            diff = calcDiff(m, d, w, h);
            for (int i = 0; i < h; ++i) {
                for (int j = 0; j < w; ++j) {
                    m.get(i).get(j).v -= d.get(i).get(j).v;
                }
            }
        }

        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                int k = 0;
                if (i != 0) k++;
                if (j != 0) k++;
                if (i < h - 1) k++;
                if (j < w - 1) k++;
                cur[m.get(i).get(j).fixed + 1] += d.get(i).get(j).v * k;
            }
        }

        return (cur[2] - cur[0]) / 2.0;
    }

    public static void main(String[] args) {
        List<List<Node>> mesh = new ArrayList<>(S);
        for (int i = 0; i < S; ++i) {
            List<Node> t = new ArrayList<>(S);
            for (int j = 0; j < S; ++j) {
                t.add(new Node(0.0, 0));
            }
            mesh.add(t);
        }

        double r = 2.0 / iter(mesh, S, S);
        System.out.printf("R = %.15f", r);
    }
}