leetcode542.cpp

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
        int m = mat.size();
        int n = mat[0].size();

        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] = -1;
                }
            }
        }


        int dr[4] = {-1, 1, 0, 0};
        int dc[4] = {0, 0, -1, 1};

       
        while (!q.empty()) {
            auto front = q.front();
            q.pop();

            int r = front.first;
            int c = front.second;

            for (int k = 0; k < 4; k++) {
                int nr = r + dr[k];
                int nc = c + dc[k];

        
                if (nr >= 0 && nr < m && nc >= 0 && nc < n && mat[nr][nc] == -1) {
                    mat[nr][nc] = mat[r][c] + 1;
                    q.push({nr, nc});
                }
            }
        }

        return mat;
    }
};