https://oj.leetcode.com/problems/spiral-matrix/

Q54.cpp

class Solution {
private:
    void spiral_r(vector<vector<int> > const& matrix,
                  vector<int> &result,
                  int m, int n, int start_m, int start_n) {
        if (m<=0 || n<=0) return;
        if (n == 1) {
            for (int i = 0; i < m; i++) result.push_back(matrix[start_m+i][start_n]);
            return;
        } else if (m == 1) {
            for (int i = 0; i < n; i++) result.push_back(matrix[start_m][start_n+i]);
            return;
        }
        for (int i = 0; i < n-1; i++) { result.push_back(matrix[start_m][i + start_n]); }
        for (int i = 0; i < m-1; i++) { result.push_back(matrix[i + start_m][n - 1 + start_n]); }
        for (int i = n - 1; i > 0; i--) { result.push_back(matrix[m - 1 + start_m][i + start_n]); }
        for (int i = m - 1; i > 0; i--) { result.push_back(matrix[i + start_m][start_n]); }
        spiral_r(matrix, result, m-2, n-2, start_m+1, start_n+1);
    }
public:
    vector<int> spiralOrder(vector<vector<int> > &matrix) {
        int m = (int)matrix.size(), n = 0;
        if (m) n = (int)matrix[0].size();
        if (!n) return vector<int>();
        vector<int> result;
        spiral_r(matrix, result, m, n, 0, 0);
        return result;
    }
};