package com.insist.entity;import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;/*** * @author SNOOPY 消除一切左递归*/
public class EliminateLeftRecursion {private static int n;// 实际输入产生式个数public static void main(String[] args) {Scanner scan = new Scanner(System.in);System.out.println("请输入产生式个数：");n = scan.nextInt();// 创建产生式数组存放输入的产生式List<Regular> regulars = new ArrayList<Regular>();for (int i = 0; i < n; i++) {Regular reg = new Regular();System.out.println("请输入产生式左部:");String left = scan.next();reg.setLeft(left);System.out.println("请输入产生式的右部:");String right = scan.next();reg.setRight(right);regulars.add(reg);}/** 测试输出------->成功 for (Regular reg: regulars) { System.out.println(reg.getLeft()+"---->"+reg.getRight()); }*/// 构造一个字符型的数组用来存放排序好的非终结符String[] Vn = new String[50];// 对所有的产生式按照一定的顺序存放Vn[0] = regulars.get(0).getLeft();// 把产生式第一个非终结符放到集合中int flag = 0;int count = 0;for (int i = 1; i < n; i++) {// 对非终结符排序并存取 1、遍历产生式数组for (int j = 0; j < i; j++) {// 如果产生式左部等于在它前面的产生式的左部if (regulars.get(i).getLeft().equals(regulars.get(j).getLeft())) {// 说明有重复的flag++;}}if (flag == 0) {// 说明没有重复，则加入非终结符数组中count++;Vn[count] = regulars.get(i).getLeft();}flag = 0;}/** 测试非终结符数组------------>成功 for (int i = 0; i < Vn.length; i++) { if(Vn[i]!=null){ System.out.println(Vn[i]); } }*/for (Regular reg : regulars) {if (reg != null) {System.out.println(reg.getLeft() + "---->" + reg.getRight());}}regulars = subFunction(regulars, Vn, count);for (Regular reg : regulars) {if (reg != null) {System.out.println(reg.getLeft() + "---->" + reg.getRight());}}}public static List<Regular> subFunction(List<Regular> regulars, String[] Vn, int count) {int flag = 0;// 判断是否存在间接左递归并转化为直接左递归for (int i = 0; i <= count; i++) {// 对每一个非终结符 迭代for (int j = 0; j < i; j++) {// 对每一个小于i的非终结符遍历for (int k = 0; k < regulars.size(); k++) // 对每一个产生式if (Vn[i].equals(regulars.get(k).getLeft())) {// i非终结符与第k产生式左边第一个字母相等-->锁定非终结符集合中的一个非终结符的产生式if (regulars.get(k).getRight().substring(0, 1).equals(Vn[j])) { // g产生式右边产生式第一个符号与第j个非终结符相等-->说明存在间接左递归for (int h = 0; h < regulars.size(); h++) {if (regulars.get(h).getLeft().equals(Vn[j])) {// 进行替换
                                    String str;str = regulars.get(k).getRight().substring(1);// 截取右边第一个以后的字符Regular reg = new Regular();reg.setLeft(regulars.get(k).getLeft());reg.setRight(regulars.get(h).getRight() + str);regulars.add(reg);}}regulars.remove(k);}}}}// 消除所有直接左递归for (int i = 0; i <= count; i++) {flag = 0;for (int j = 0; j < regulars.size(); j++) {// 判断是否存在直接左递归if (regulars.get(j).getLeft().equals(Vn[i])) {System.out.println(regulars.get(j).getLeft() + " ======= " + Vn[i]);if (regulars.get(j).getLeft().equals(regulars.get(j).getRight().substring(0, 1))) {System.out.println("消除间接左递归后存在直接左递归");flag++;}}}if (flag != 0) {// 存在直接左递归for (int j = 0; j < regulars.size(); j++) {if (regulars.get(j).getLeft().equals(Vn[i])) {// 寻找与存在直接左递归的非终结符左部相同的的产生式if (regulars.get(j).getLeft().equals(regulars.get(j).getRight().substring(0, 1))) {// 直接左递归的产生式String str = regulars.get(j).getRight().substring(1);String temp = regulars.get(j).getLeft();String temp1 = "'";regulars.get(j).setLeft(temp + temp1);regulars.get(j).setRight(str + regulars.get(j).getLeft());Regular reg = new Regular();reg.setLeft(regulars.get(j).getLeft());reg.setRight("ε");regulars.add(reg);} else {String temp = regulars.get(j).getLeft();String temp1 = "'";temp = temp + temp1;regulars.get(j).setRight(regulars.get(j).getRight() + temp);}}}}}return regulars;}
}

package com.insist.entity;import java.io.Serializable;/*** * @author SNOOPY**/
public class Regular implements Serializable {private static final long serialVersionUID = 1L;private String right;// 定义产生式右部private String left;// 定义产生式左部public String getRight() {return right;}public void setRight(String right) {this.right = right;}public String getLeft() {return left;}public void setLeft(String left) {this.left = left;}
}