Java: 

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public List<Integer> spiralOrder(int[][] matrix) {
    List<Integer> res = new ArrayList<>();
    if (matrix.length == 0 || matrix[0].length == 0) return res;
    
    int left = 0;
    int right = matrix[0].length - 1;
    int top = 0;
    int bottom = matrix.length - 1;
    
    while (true) {
        for (int i = left; i <= right; ++i) res.add(matrix[top][i]);
        top++;
        if (left > right || top > bottom) break;
        
        for (int i = top; i <= bottom; ++i) res.add(matrix[i][right]);
        right--;
        if (left > right || top > bottom) break;
        
        for (int i = right; i >= left; --i) res.add(matrix[bottom][i]);
        bottom--;
        if (left > right || top > bottom) break;
        
        for (int i = bottom; i >= top; --i) res.add(matrix[i][left]);
        left++;
        if (left > right || top > bottom) break;
        
    }
    return res;
}