542-01 Matrix

tags: Medium

Question

https://leetcode.com/problems/01-matrix/

Key

• BFS
  

• use DP twice

Reference

BFS Solution

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

DP solution

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
        int rows = matrix.size();
        if (rows == 0) 
            return matrix;
        int cols = matrix[0].size();
        vector<vector<int>> dist(rows, vector<int> (cols, INT_MAX - 100000));

        //First pass: check for left and top
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (matrix[i][j] == 0) {
                    dist[i][j] = 0;
                } else {
                    if (i > 0)
                        dist[i][j] = min(dist[i][j], dist[i - 1][j] + 1);
                    if (j > 0)
                        dist[i][j] = min(dist[i][j], dist[i][j - 1] + 1);
                }
            }
        }
        //Second pass: check for bottom and right
        for (int i = rows - 1; i >= 0; i--) {
            for (int j = cols - 1; j >= 0; j--) {
                if (i < rows - 1)
                    dist[i][j] = min(dist[i][j], dist[i + 1][j] + 1);
                if (j < cols - 1)
                    dist[i][j] = min(dist[i][j], dist[i][j + 1] + 1);
            }
        }
        return dist;
    }
};