0059.cpp

class Solution {
    pair<int, int> offset[4] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
    int limit_offset[4] = {1, -1, -1, 1};
    int limit[4] = {0, 0, 0, 0};

    int &m = limit[2], &n = limit[1];

    bool valid(int i, int j) { return i >= limit[0] && i <= m && j >= limit[3] && j <= n; }

  public:
    vector<vector<int>> generateMatrix(int dim) {
        vector<vector<int>> res(dim, vector<int>(dim));
        int direction = 0;
        int cnt = 0;
        int size;
        int i = 0, j = 0;

        m = n = dim - 1;
        size = (m + 1) * (n + 1);

        while (true) {
            res[i][j] = ++cnt;
            if (cnt == size) break;

            if (!valid(i + offset[direction].first, j + offset[direction].second)) {
                limit[direction] += limit_offset[direction];
                direction = (direction + 1) % 4;
            }

            i += offset[direction].first;
            j += offset[direction].second;
        }

        return res;
    }
};