pacific-atlantic-water-flow（不错）

https://leetcode.com/problems/pacific-atlantic-water-flow/

// 其实，这道题目可以参考前面的那道：Trapping Rain Water II
// 可以看我blog上面的解法：
// http://www.cnblogs.com/charlesblc/p/5920258.html

// 注意Java set的操作，retainAll 交集，removeAll差集
// 开始set里面放的是int[]，但是发现比较的时候，都被认为不一样，所以需要换成Block

// 第一次提交还报了一个Runtime Error，因为没有考虑row或者column为0的情况

public class Solution {

    private class Block {
        public int row;
        public int col;
        public int height;
        public Block(int r, int c, int h) {
            row = r;
            col = c;
            height = h;
        }

        public int hashCode() {
            return row + col + height;
        }

        public boolean equals(Object obj) {
            if (obj instanceof Block) {
                Block bk = (Block)obj;
                return bk.row == row && bk.col == col;
            }
            return false;
        }

    }

    public List<int[]> pacificAtlantic(int[][] matrix) {
        List<int[]> ret = new ArrayList<int[]>();

        int r = matrix.length;
        if (r == 0) {
            return ret;
        }
        int c = matrix[0].length;
        if (c == 0) {
            return ret;
        }

        Set<Block> pSt = new HashSet<Block>();
        Set<Block> aSt = new HashSet<Block>();

        Queue<Block> qe = new PriorityQueue<Block>(1,
                new Comparator<Block>() {
                    @Override
                    public int compare(Block o1, Block o2) {
                        return o1.height - o2.height;
                    }
                }
        );
        boolean[][] visited = new boolean[r][c];
        int[][] dirs = {{-1,0}, {0, 1}, {1, 0}, {0, -1}};

        for (int i=0; i<r; i++) {
            Block bk = new Block(i, 0, matrix[i][0]);
            pSt.add(bk);
            //System.out.printf("add pset: %d, %d\n", tmp[0], tmp[1]);
            qe.offer(bk);
            visited[i][0] = true;
        }
        for (int i=0; i<c; i++) {
            Block bk = new Block(0, i, matrix[0][i]);
            pSt.add(bk);
            //System.out.printf("add pset: %d, %d\n", tmp[0], tmp[1]);
            qe.offer(bk);
            visited[0][i] = true;
        }

        while (!qe.isEmpty()) {
            Block bk = qe.poll();
            for (int i=0; i<4; i++) {
                int nr = bk.row + dirs[i][0];
                int nc = bk.col + dirs[i][1];
                if (nr < 0 || nr >= r || nc < 0 || nc >= c || visited[nr][nc]) {
                    continue;
                }
                visited[nr][nc] = true;
                if (matrix[nr][nc] >= bk.height) {
                    Block nbk = new Block(nr, nc, matrix[nr][nc]);
                    pSt.add(nbk);
                    //System.out.printf("add pset new: %d, %d\n", tmp[0], tmp[1]);
                    qe.offer(nbk);
                }
            }
        }

        // 重新初始化，计算atlantic
        qe.clear();
        visited = new boolean[r][c];

        for (int i=0; i<r; i++) {
            Block bk = new Block(i, c-1, matrix[i][c-1]);
            aSt.add(bk);
            //System.out.printf("add aset: %d, %d\n", tmp[0], tmp[1]);
            qe.offer(bk);
            visited[i][c-1] = true;
        }
        for (int i=0; i<c; i++) {
            Block bk = new Block(r-1, i, matrix[r-1][i]);
            aSt.add(bk);
            //System.out.printf("add aset: %d, %d\n", tmp[0], tmp[1]);
            qe.offer(bk);
            visited[r-1][i] = true;
        }

        while (!qe.isEmpty()) {
            Block bk = qe.poll();
            for (int i=0; i<4; i++) {
                int nr = bk.row + dirs[i][0];
                int nc = bk.col + dirs[i][1];
                if (nr < 0 || nr >= r || nc < 0 || nc >= c || visited[nr][nc]) {
                    continue;
                }
                visited[nr][nc] = true;
                if (matrix[nr][nc] >= bk.height) {
                    Block nbk = new Block(nr, nc, matrix[nr][nc]);
                    aSt.add(nbk);
                    //System.out.printf("add aset new: %d, %d\n", tmp[0], tmp[1]);
                    qe.offer(nbk);
                }
            }
        }

        //System.out.printf("pset length: %d\n", pSt.size());
        //System.out.printf("aset length: %d\n", aSt.size());
        pSt.retainAll(aSt);
        System.out.printf("set length: %d\n", pSt.size());

        Iterator itr = pSt.iterator();
        while (itr.hasNext()) {
            Block bk = (Block)itr.next();
            int[] val = {bk.row, bk.col};
            ret.add(val);
        }
        return ret;
    }

}