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class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> res;
        if (matrix.empty() || matrix.size() == 0 || matrix[0].empty() || matrix[0].size() == 0) return res;
        int rowBegin = 0;
        int rowEnd = matrix.size() - 1;
        int colBegin = 0;
        int colEnd = matrix[0].size() - 1;
        
        // loop over 2 ~ 4 steps
        while (rowBegin <= rowEnd && colBegin <= colEnd) {
            for (int i = colBegin; i <= colEnd; i++) {
                res.push_back(matrix[rowBegin][i]);
            }
            rowBegin++;   // all elements in first row ended
            
            for (int i = rowBegin; i <= rowEnd; i++) {
                res.push_back(matrix[i][colEnd]);
            }
            colEnd--;   // all elements in last row ended
            
            if (rowBegin <= rowEnd) {
                for (int i = colEnd; i >= colBegin; i--) {
                    res.push_back(matrix[rowEnd][i]);
                }
            }
            rowEnd--;
            
            if (colBegin <= colEnd) {
                for (int i = rowEnd; i >= rowBegin; i--) {
                    res.push_back(matrix[i][colBegin]);
                }
            }
            colBegin++;
        }
        
        return res;
    }
};