[LeetCode] Spiral Matrix

The idea is just to add the elements in the spiral order. First the up-most row (u), then the right-most column (r), then the down-most row (d), and finally the left-most column (l). After finishing a row or a column, update the corresponding variable to continue the process.

The code is as follows.

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        if (matrix.empty()) return {};
        int m = matrix.size(), n = matrix[0].size();
        vector<int> spiral(m * n);
        int u = 0, d = m - 1, l = 0, r = n - 1, k = 0;
        while (true) {
            // up
            for (int col = l; col <= r; col++) spiral[k++] = matrix[u][col];
            if (++u > d) break;
            // right
            for (int row = u; row <= d; row++) spiral[k++] = matrix[row][r];
            if (--r < l) break;
            // down
            for (int col = r; col >= l; col--) spiral[k++] = matrix[d][col];
            if (--d < u) break;
            // left
            for (int row = d; row >= u; row--) spiral[k++] = matrix[row][l];
            if (++l > r) break;
        }
        return spiral;
    }
};