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>() {@Overridepublic 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;}}