909. 蛇梯棋

题目：

 

思路：

【1】广度优先搜索

代码展示：

【1】广度优先搜索

//时间4 ms 击败 69.78%
//内存42.3 MB 击败 49.22%
class Solution {
    public int snakesAndLadders(int[][] board) {
        int n = board.length;
        boolean[] vis = new boolean[n * n + 1];
        Queue<int[]> queue = new LinkedList<int[]>();
        queue.offer(new int[]{1, 0});
        while (!queue.isEmpty()) {
            int[] p = queue.poll();
            for (int i = 1; i <= 6; ++i) {
                int nxt = p[0] + i;
                if (nxt > n * n) { // 超出边界
                    break;
                }
                int[] rc = id2rc(nxt, n); // 得到下一步的行列
                if (board[rc[0]][rc[1]] > 0) { // 存在蛇或梯子
                    nxt = board[rc[0]][rc[1]];
                }
                if (nxt == n * n) { // 到达终点
                    return p[1] + 1;
                }
                if (!vis[nxt]) {
                    vis[nxt] = true;
                    queue.offer(new int[]{nxt, p[1] + 1}); // 扩展新状态
                }
            }
        }
        return -1;
    }

    public int[] id2rc(int id, int n) {
        int r = (id - 1) / n, c = (id - 1) % n;
        if (r % 2 == 1) {
            c = n - 1 - c;
        }
        return new int[]{n - 1 - r, c};
    }
}


//时间3 ms 击败 93.46%
//内存42.1 MB 击败 60.13%
class Solution {
    public int snakesAndLadders(int[][] board) {
        int n = board.length;
        int[] arr = new int[n * n + 1];
        int cnt = 1;
        for(int i = n - 1; i >= 0; i--) {
            if((n - 1 - i) % 2 == 0) {
                for(int j = 0; j < n; j++) {
                    arr[cnt++] = board[i][j];
                }
            }else {
                for(int j = n - 1; j >= 0; j--) {
                    arr[cnt++] = board[i][j];
                }
            }
        }
        int step = 0;
        int[] vis = new int[n * n + 1];
        Queue<Integer> queue = new LinkedList<>();
        queue.offer(1);
        vis[1] = 1;
        while(!queue.isEmpty()) {
            step++;
            int size = queue.size();
            for(int i = 0; i < size; i++) {
                int start = queue.poll();
                for(int j = 1; j <= 6; j++) {
                    if(start + j > n * n) {
                        break;
                    }
                    int end = start + j;
                    if(arr[end] != -1)
                        end = arr[end];
                    if(vis[end] != 0)
                        continue;
                    queue.offer(end);
                    vis[end] = 1;
                }
                if(vis[n * n] == 1) {
                    return step;
                }
            }
        }
        return -1;
    }
}