第二章搜索（三）

雙向廣搜

例題：190. 字串變換

有錯…

import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Scanner;

public class Main {
    static int N = 6;
    static int n;
    static String[] a = new String[N];
    static String[] b = new String[N];

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String A = sc.next();
        String B = sc.next();
        while (sc.hasNext()) {
            a[n] = sc.next();
            b[n] = sc.next();
            n++;
        }
        int step = bfs(A, B);
        if (step > 10) {
            System.out.println("NO ANSWER!");
        } else {
            System.out.println(step);
        }
    }

    private static int bfs(String A, String B) {
        LinkedList<String> qa = new LinkedList<>();
        LinkedList<String> qb = new LinkedList<>();
        Map<String, Integer> da = new HashMap<>();
        Map<String, Integer> db = new HashMap<>();
        qa.add(A);
        qb.add(B);
        da.put(A, 0);
        db.put(B, 1);
        while (qa.size() > 0 && qb.size() > 0) {
            if (da.get(qa.peek()) + db.get(qb.peek()) > 10) return 11;
            int t;
            if (qa.size() <= qb.size()) t = extend(qa, da, db, a, b);
            else t = extend(qb, db, da, b, a);
            if (t != -1) return t;
        }
        return 11;
    }

    private static int extend(LinkedList<String> q, Map<String, Integer> da, Map<String, Integer> db, String[] a, String[] b) {
        String t = q.poll();
        for (int i = 0; i < t.length(); i++) {
            for (int j = 0; j < n; j++) {
                if (!t.substring(i, a[j].length()).equals(a[j])) continue;
                String r = t.substring(0, i) + b[j] + t.substring(i + a[j].length());
                if (db.containsKey(r)) return db.get(r) + da.get(t);
                if (da.containsKey(r)) continue;
                da.put(r, da.get(t) + 1);
                q.add(r);
            }
        }
        return -1;
    }
}