图计算中的图剪枝算法是什么？请解释其作用和常用方法。

import java.util.Arrays;
public class PageRank {
    public static void main(String[] args) {
        // 网页链接矩阵
        int[][] linkMatrix = {
            {0, 1, 1, 0},
            {1, 0, 1, 1},
            {1, 0, 0, 1},
            {0, 1, 1, 0}
        };
        // 网页数量
        int numPages = linkMatrix.length;
        // 初始化PageRank值
        double[] pageRank = new double[numPages];
        Arrays.fill(pageRank, 1.0 / numPages);
        // 迭代计算PageRank值
        double dampingFactor = 0.85; // 阻尼系数
        int numIterations = 10; // 迭代次数
        for (int i = 0; i < numIterations; i++) {
            double[] newPageRank = new double[numPages];
            for (int j = 0; j < numPages; j++) {
                for (int k = 0; k < numPages; k++) {
                    if (linkMatrix[k][j] == 1) {
                        newPageRank[j] += pageRank[k] / countOutlinks(linkMatrix, k);
                    }
                }
                newPageRank[j] = (1 - dampingFactor) / numPages + dampingFactor * newPageRank[j];
            }
            pageRank = newPageRank;
        }
        // 输出PageRank值
        for (int i = 0; i < numPages; i++) {
            System.out.println("Page " + i + ": " + pageRank[i]);
        }
    }
    // 计算指定网页的出链数量
    private static int countOutlinks(int[][] linkMatrix, int page) {
        int count = 0;
        for (int i = 0; i < linkMatrix[page].length; i++) {
            if (linkMatrix[page][i] == 1) {
                count++;
            }
        }
        return count;
    }
}