leetcode Spiral Matrix

class Solution {  
 public:  
  vector spiralOrder(vector > &matrix) {  
    vector res;  
    int rownum = matrix.size(), colnum = matrix.size()   matrix[0].size() : 0, i, j;  
    if (rownum == 0 || colnum == 0)  
      return res;  
    int rowstart = 0, rowend = rownum - 1, colstart = 0, colend = colnum - 1;  
    while (rowstart <= rowend && colstart <= colend) {  
      for (i = colstart; i <= colend; ++i)  
        res.push_back(matrix[rowstart][i]);  
      for (i = rowstart + 1; i <= rowend; ++i)  
        res.push_back(matrix[i][colend]);  
      if (rowstart != rowend)  
        for (i = colend - 1; i >





= colstart; --i)  
          res.push_back(matrix[rowend][i]);  
      if (colstart != colend)  
        for (i = rowend - 1; i > rowstart; --i)  
          res.push_back(matrix[i][colstart]);  
      ++rowstart;  
      ++colstart;  
      --rowend;  
      --colend;  
    }  
    return res;  
  }  
};