leetcode 542. 01 Matrix 、663. Walls and Gates(lintcode) 、773. Sliding Puzzle 、803. Shortest Distance from All Buildings

542. 01 Matrix

https://www.cnblogs.com/grandyang/p/6602288.html

将所有的1置为INT_MAX，然后用所有的0去更新原本位置为1的值。

最短距离肯定使用bfs。

每次更新了值的地方还要再加入队列中 。

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
        int m = matrix.size(),n = matrix[0].size();
        queue<pair<int,int>> q;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(matrix[i][j] == 0)
                    q.push(make_pair(i,j));
                else
                    matrix[i][j] = INT_MAX;
            }
        }
        while(!q.empty()){
            auto coordinate = q.front();
            q.pop();
            int x = coordinate.first,y = coordinate.second;
            for(auto dir : dirs){
                int new_x = x + dir[0];
                int new_y = y + dir[1];
                if(new_x < 0 || new_x >= matrix.size() || new_y < 0 || new_y >= matrix[0].size() || matrix[new_x][new_y] <= matrix[x][y] + 1)
                    continue;
                matrix[new_x][new_y] = matrix[x][y] + 1;
                q.push(make_pair(new_x,new_y));
            }
        }
        return matrix;
    }
private:
    vector<vector<int>> dirs{{-1,0},{1,0},{0,-1},{0,1}};
};

 

663. Walls and Gates

https://www.cnblogs.com/grandyang/p/5285868.html

这个题跟542. 01 Matrix很像，主要利用bfs。先把所有的0位置放入队列中，然后通过0更新周围的位置达到更新所有的位置。

class Solution {
public:
    /**
     * @param rooms: m x n 2D grid
     * @return: nothing
     */
    void wallsAndGates(vector<vector<int>> &rooms) {
        // write your code here
        queue<pair<int,int>> q;
        for(int i = 0;i < rooms.size();i++){
            for(int j = 0;j < rooms[0].size();j++){
                if(rooms[i][j] == 0)
                    q.push(make_pair(i,j));
            }
        }
        while(!q.empty()){
            int x = q.front().first;
            int y = q.front().second;
            q.pop();
            for(auto dir : dirs){
                int x_new = x + dir[0];
                int y_new = y + dir[1];
                if(x_new < 0 || x_new >= rooms.size() || y_new < 0 || y_new >= rooms[0].size() || rooms[x_new][y_new] < rooms[x][y] + 1)
                    continue;
                rooms[x_new][y_new] = rooms[x][y] + 1;
                q.push(make_pair(x_new,y_new));
            }
        }
        return;
    }
private:
    vector<vector<int>> dirs{{-1,0},{1,0},{0,-1},{0,1}};
};

 

773. Sliding Puzzle

https://www.cnblogs.com/grandyang/p/8955735.html

queue里面存储的是每次变换后的board的样子和当前新的board中0所在的坐标。

将队列一次清空完了之后才能更新res，因为这就是一次变换。

用set相当于决定什么时候停止循环

class Solution {
public:
    int slidingPuzzle(vector<vector<int>>& board) {
        int res = 0,m = board.size(),n = board[0].size();
        set<vector<vector<int>>> visited;
        vector<vector<int>> correct{{1,2,3},{4,5,0}};
        queue<pair<vector<vector<int>>,vector<int>>> q;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(board[i][j] == 0){
                    vector<int> tmp;
                    tmp.push_back(i);
                    tmp.push_back(j);
                    q.push(make_pair(board,tmp));
                }
            }
        }
        while(!q.empty()){
            for(int i = q.size();i > 0;i--){
                int x = q.front().second[0];
                int y = q.front().second[1];
                vector<vector<int>> board_new = q.front().first;
                if(board_new == correct)
                    return res;
                visited.insert(board_new);
                q.pop();
                for(auto dir : dirs){
                    int new_x = x + dir[0];
                    int new_y = y + dir[1];
                    if(new_x < 0 || new_x >= m || new_y < 0 || new_y >= n)
                        continue;
                    vector<vector<int>> cand = board_new;
                    swap(cand[x][y],cand[new_x][new_y]);
                    if(visited.find(cand) != visited.end())
                        continue;
                    vector<int> tmp;
                    tmp.push_back(new_x);
                    tmp.push_back(new_y);
                    q.push(make_pair(cand,tmp));
                }
            }
            res++;
        }
        return -1;
    }
private:
    vector<vector<int>> dirs{{-1,0},{1,0},{0,-1},{0,1}};
};

注意犯的错误：

 for(int i = q.size();i > 0;i--)不能写成for(int i = 0;i < q.size();i++)，因为q的size在循环一次后，大小是缩小的，这样不能保证正确的次数。

 

 

 

803. Shortest Distance from All Buildings

https://www.cnblogs.com/grandyang/p/5297683.html

到所有建筑物的距离和最小，其实也就等于到每个建筑物最小距离的和。

以每个建筑物做bfs求最小的距离，然后用一个dis的vector保存，每当增加一个建筑物，在相应位置叠加当前建筑物的最小距离即可。

有可能有些点无法访问到某一些建筑物，所以通过一个count矩阵存储每个位置能访问到的建筑物的个数，最后再判断能否达到。

class Solution {
public:
    /**
     * @param grid: the 2D grid
     * @return: the shortest distance
     */
    int shortestDistance(vector<vector<int>> &grid) {
        // write your code here
        int m = grid.size(),n = grid[0].size();
        int res = INT_MAX,building = 0;
        vector<vector<int>> dist(m,vector<int>(n,0));
        vector<vector<int>> count = dist;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(grid[i][j] == 1){
                    building++;
                    queue<pair<int,int>> q;
                    vector<vector<bool>> visited(m,vector<bool>(n,false));
                    q.push(make_pair(i,j));
                    int pace = 0;
                    while(!q.empty()){
                        pace++;
                        for(int i = q.size();i > 0;i--){
                            int x = q.front().first;
                            int y = q.front().second;
                            q.pop();
                            for(auto dir : dirs){
                                int new_x = x + dir[0];
                                int new_y = y + dir[1];
                                if(new_x < 0 || new_x >= m || new_y < 0 || new_y >= n || visited[new_x][new_y]== true || grid[new_x][new_y] != 0)
                                    continue;
                                dist[new_x][new_y] += pace;
                                count[new_x][new_y] += 1;
                                visited[new_x][new_y] = true;
                                q.push(make_pair(new_x,new_y));
                            }
                        }
                    }
                }
            }
        }
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(grid[i][j] == 0 && count[i][j] == building)
                    res = min(res,dist[i][j]);
            }
        }
        return res == INT_MAX ? -1 : res;
    }
private:
    vector<vector<int>> dirs{{-1,0},{1,0},{0,-1},{0,1}};
};

 

另一种写法：

class Solution {
public:
    /**
     * @param grid: the 2D grid
     * @return: the shortest distance
     */
    int shortestDistance(vector<vector<int> > &grid) {
        // write your code here
        int m = grid.size(),n = grid[0].size();
        vector<vector<int> > path(m,vector<int>(n,0));
        vector<vector<int> > count(m,vector<int>(n,0));
        int building = 0;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(grid[i][j] == 1){
                    building++;
                    vector<vector<bool> > visited(m,vector<bool>(n,false));
                    search(grid,i,j,path,count,visited);
                }
            }
        }
        int res = INT_MAX;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(grid[i][j] == 0 && count[i][j] == building)
                    res = min(res,path[i][j]);
            }
        }
        return res == INT_MAX ? -1 : res;
    }
    void search(vector<vector<int> > grid,int x,int y,vector<vector<int> >& path,vector<vector<int> >& count,vector<vector<bool> > visited){
        queue<pair<int,int> > q;
        q.push(make_pair(x,y));
        int pace = 0;
        while(!q.empty()){
            pace++;
            for(int i = q.size();i > 0;i--){
                int x0 = q.front().first;
                int y0 = q.front().second;
                q.pop();
                //visited[x0][y0] = true;
                for(int j = 0;j < dirs.size();j++){
                    int new_x = x0 + dirs[j][0];
                    int new_y = y0 + dirs[j][1];
                    if(new_x < 0 || new_x >= path.size() || new_y < 0 || new_y >= path[0].size() || visited[new_x][new_y] || grid[new_x][new_y] != 0)
                        continue;
                    path[new_x][new_y] += pace;
                    count[new_x][new_y]++;
                    visited[new_x][new_y] = true;
                    q.push(make_pair(new_x,new_y));
                }
            }
        }
    }
private:
    vector<vector<int> > dirs{{-1,0},{1,0},{0,-1},{0,1}};
};


//visited[new_x][new_y] = true;只能放在新生成的坐标的位置