BFS

BFS找最短路径用level order，有模版。
图上的搜素记得考虑用户visited数组

visited[][] 
queue.add(start)
visited.add(start)
while(queue not empty) {
  int size = queue.size
  for (0 - size) {
  }
}

675. Cut Off Trees for Golf Event

You are asked to cut off trees in a forest for a golf event. The forest is represented as a non-negative 2D map, in this map:

0 represents the obstacle can't be reached.
1 represents the ground can be walked through.
The place with number bigger than 1 represents a tree can be walked through, and this positive number represents the tree's height.
You are asked to cut off all the trees in this forest in the order of tree's height - always cut off the tree with lowest height first. And after cutting, the original place has the tree will become a grass (value 1).

You will start from the point (0, 0) and you should output the minimum steps you need to walk to cut off all the trees. If you can't cut off all the trees, output -1 in that situation.

You are guaranteed that no two trees have the same height and there is at least one tree needs to be cut off.

Example 1:
Input:
[
[1,2,3],
[0,0,4],
[7,6,5]
]
Output: 6

这道题没做出来的原因是没想清楚如何手动算出结果。从起点开始走，找出当前最矮的树，走过去，再找到第二矮的树走过去，累加步数。所以想到用heap找到当前最矮的树，然后用bfs算出当前点到树的最短路径。起点也要加到队列里。砍完不用修改传进来的list of list，比如从4走到7，说明没有高度是5，6的树还存在了。

class Solution {
    // move 4 directions
    int[] dX = {0, 0, -1, 1};
    int[] dY = {1, -1, 0 , 0};
    public int cutOffTree(List> forest) {
        int m = forest.size();
        int n = forest.get(0).size();
        Comparator com = (p1, p2) -> p1.val - p2.val;
        PriorityQueue queue = new PriorityQueue<>(com); // every time minimum value is poped
        // add all postion and its value into queue
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                // not add (0, 0) into queue
                if (forest.get(i).get(j) > 1) {
                    Position p = new Position(i, j, forest.get(i).get(j));
                    queue.add(p);
                }
            }
        }
        int res = 0;
        // start from 0, 0
        Position start = new Position(0, 0, 0);
        while (!queue.isEmpty()) {
            Position end = queue.poll();
            int steps = helper(start, end, forest, m, n);
            if (steps < 0) {
                return -1;
            }
            res += steps;
            start = end;
        }
        
        return res;
    }
    // return the minumum steps from start to end, using BFS
    private int helper(Position start, Position end, List> forest, int m, int n) {
        // record visited array, using 2-d array
        int[][] visited = new int[m][n];
        Queue queue = new LinkedList<>();
        queue.add(start);
        visited[start.x][start.y] = 1;
        int step = 0;        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                Position cur = queue.poll();
                if (cur.x == end.x && cur.y == end.y) {
                    return step;
                }
                // go to four directions
                for (int j = 0; j < 4; j++) {
                    // add qulified position into queue
                    int new_x = cur.x + dX[j], new_y = cur.y + dY[j];
                    if (new_x >= 0 && new_x < m && new_y >= 0 && new_y < n
                        && forest.get(new_x).get(new_y) > 0
                        && visited[new_x][new_y] != 1) {
                        Position p = new Position(new_x, new_y, forest.get(new_x).get(new_y));
                        queue.add(p);
                        visited[new_x][new_y] = 1;
                    }
                }
            }
            step++;
        }
        return -1;
    }

}

class Position {
    int x; 
    int y;
    int val;
    public Position (int x, int y, int val) {
        this.x = x;
        this.y = y;
        this.val = val;
    }
}

742. Closest Leaf in a Binary Tree

见DFS

286. Walls and Gates

-1 - A wall or an obstacle.
0 - A gate.
INF - Infinity means an empty room.
For example, given the 2D grid:
INF -1 0 INF
INF INF INF -1
INF -1 INF -1
0 -1 INF INF
After running your function, the 2D grid should be:
3 -1 0 1
2 2 1 -1
1 -1 2 -1
0 -1 3 4

DFS BFS都可以做，需要注意的是以门为起点去向下延伸，而不是以房间开始

class Solution {
    public void wallsAndGates(int[][] rooms) {
        Queue queue = new LinkedList<>();
        for (int i = 0; i < rooms.length; i++) {
            // push gate into queue
            for (int j = 0; j < rooms[i].length; j++) {
                if (rooms[i][j] == 0) {
                    queue.offer(new int[]{i, j});
                }
            }
        }
        int[] dx = {0, 0, -1, 1};
        int[] dy = {1, -1, 0, 0};
        while (!queue.isEmpty()) {
            int[] cor = queue.poll();
            int i = cor[0], j = cor[1];
            for (int idx = 0; idx < 4; idx++) {
                if (i + dx[idx] >= 0 && i + dx[idx] <= rooms.length - 1 && j + dy[idx] >= 0 && j +                                                  dy[idx] <= rooms[0].length - 1 && rooms[i + dx[idx]][j + dy[idx]] == Integer.MAX_VALUE) {
                    rooms[i + dx[idx]][j + dy[idx]] = rooms[i][j] + 1;
                    queue.offer(new int[]{i + dx[idx], j + dy[idx]});
                }
            }
        }
    }
}