【At Coder begin 345】[D - Tiling] 回溯

题目链接：<problem D>

解法：

1. 比较简单。看代码。

   import java.io.IOException;
   import java.io.InputStreamReader;
   import java.io.StreamTokenizer;
   import java.util.Set;
   import java.util.TreeSet;

   public class Main {
   static int n, h, w;
   static int[][] ties;

    static boolean[][] writed ;
   
    static boolean[] used;
   
    static boolean over = false;
   
    static Set<Integer> set = new TreeSet<>();
    public static void main(String[] args) throws IOException {
        StreamTokenizer st = new StreamTokenizer(new InputStreamReader(System.in));
        st.nextToken();
        n = (int) st.nval;
        st.nextToken();
        h = (int) st.nval;
        st.nextToken();
        w = (int) st.nval;
   
        ties = new int[n][2];
        for (int i = 0; i < n; i++) {
            st.nextToken();
            int a = (int) st.nval;
            st.nextToken();
            int b = (int) st.nval;
            ties[i][0] = a;
            ties[i][1] = b;
        }
        writed = new boolean[h][w];
        used = new boolean[n];
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                set.add(i*w+j);
            }
        }
        dfs(0,0,0);
        System.out.println(over?"Yes":"No");
    }
   
    public static boolean canplace(int i,int topx,int topy,int dir){
        int r = ties[i][dir];
        int c = ties[i][1-dir];
        for(int x=topx;x<topx+r;x++){
            if( x<0 || x >=h) {
                return false;
            }
            for(int y=topy;y<topy+c;y++){
                if( y<0 || y>=w) {
                    return false;
                }
                if(writed[x][y]) {
                    return false;
                }
            }
        }
        return true;
    }
   
    public static void place(int i,int topx,int topy,int dir){
        int r = ties[i][dir];
        int c = ties[i][1-dir];
        for(int x=topx;x<topx+r;x++){
            for(int y=topy;y<topy+c;y++){
               writed[x][y] = true;
               set.remove(x*w + y);
            }
        }
        used[i] = true;
    }
   
    public static  boolean check() {
        if( set.isEmpty()) {
            return true;
        }
        return false;
    }
   
    public static void replace(int i,int topx,int topy,int dir) {
        int r = ties[i][dir];
        int c = ties[i][1-dir];
        for(int x=topx;x<topx+r;x++){
            for(int y=topy;y<topy+c;y++){
                writed[x][y] = false;
                set.add(x*w + y);
            }
        }
        used[i] = false;
    }
   
    public static void dfs(int depth,int topx,int topy) {
        if(over) {
            return;
        }
   
        if( check()){
            over = true;
            return;
        }
   
        for(int i=0;i<n;i++){
            for(int dir=0;dir<=1;dir++) {
                if (!used[i] && canplace(i, topx, topy, dir)) {
                    place(i, topx, topy, dir);
                    if(check()){
                        over = true;
                        return;
                    }
                    int[] next = update();
                    dfs(depth + 1, next[0],next[1]);
                    replace(i, topx, topy, dir);
                }
            }
        }
    }
   
    public static int[] update() {
        int u = set.stream().findFirst().orElse(0);
        return new int[]{u/w,u%w};
    }
   

   }