Spiral Order Print I

Traverse an N * N 2D array in spiral order clock-wise starting from the top left corner. Return the list of traversal sequence.

Assumptions

• The 2D array is not null and has size of N * N where N >= 0

Examples

{ {1, 2, 3},

{4, 5, 6},

{7, 8, 9} }

the traversal sequence is [1, 2, 3, 6, 9, 8, 7, 4, 5]

Solution: Recursive print layer by layer

Base Case:

One or none entries in the center

Recursive rule:

For each layer, add

1. .top left to top right - 1

2. top right to bottom right - 1

3. bottom right - bottom left - 1

4. bottom left to top left - 1

    public void spiralTrack(int[][] input, List<Integer> result, int offset, int size){
      if(size == 0) return;
      if (size == 1){
        result.add(input[offset][offset]);
        return;
      }
      for (int i = 0; i < size - 1; i++){ //top left to top right - 1
        result.add(input[offset][offset + i]);
      }
      for (int i = 0; i < size - 1; i++){
        result.add(input[i + offset][offset + size - 1]); //top right to bottom right -1 
      }
      for ( int i = size - 1; i >= 1; i--){
        result.add(input[offset + size - 1][offset + i]); //bottom right to bottom left - 1
      }
      for (int i = size - 1; i >= 1; i--){ //bottom left to top left - 1
        result.add(input[offset + i][offset]);
      }
      spiralTrack(input, result, offset + 1, size - 2);
  }