Leetcode 542 Solution

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
        int m = matrix.size();
        int 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 << 16) + j, 0));
                matrix[i][j] = -1;
            }

        int filled = 0;
        int total = m * n;
        while (filled < total)
        {
            const auto& pt = q.front();
            int i = pt.first >> 16;
            int j = pt.first & 0x0FFFF;
            int dist = pt.second;
            q.pop();

            if (matrix[i][j] != -1)
                continue;

            matrix[i][j] = dist;
            filled++;

            if (i > 0 && matrix[i - 1][j] == -1)
                q.push(make_pair(((i - 1) << 16) + j, dist + 1));

            if (i < m - 1 && matrix[i + 1][j] == -1)
                q.push(make_pair(((i + 1) << 16) + j, dist + 1));

            if (j > 0 && matrix[i][j - 1] == -1)
                q.push(make_pair((i << 16) + j - 1, dist + 1));

            if (j < n - 1 && matrix[i][j + 1] == -1)
                q.push(make_pair((i << 16) + j + 1, dist + 1));
        }

        return matrix;
    }
};