885. Spiral Matrix III

O(max(R, C)²) time O(1) space
思路就是按照旋转方向走一遍即可，出界也照走，只有在界内才放到res里

class Solution {
public:
    vector<vector<int>> spiralMatrixIII(int R, int C, int r0, int c0) {
        vector<vector<int>> res = {{r0, c0}};
        int dr[] = {0, 1, 0, -1}, dc[] = {1, 0, -1, 0};
        int sz = 1, dir = 0, r = r0, c = c0;
        while (res.size() < R * C) {
            for (int i = 0; i < sz; ++i) {
                r += dr[dir];
                c += dc[dir];
                if (0 <= r && r < R && 0 <= c && c < C) {
                    res.push_back({r, c});
                }
            }
            dir = (dir + 1) % 4; // 方向每次转一下
            sz += !(dir & 1); // 在每个方向上走的步数，每两个方向加一，即dir模2为0时加一
        }
        return res;
    }
};