059-spiral-matrix-ii

Question

https://leetcode.com/problems/spiral-matrix-ii/description/

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

Example:

Given n = 3,
You should return the following matrix:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

Thought Process

1. Very similar to 054-Spiral Matrix, we can reuse very much the same code
2. Fill the top, right, bottom, and left
3. Time complexity O(n^2)
4. Space complexity O(n^2), O(1) extra

Solution

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] result = new int[n][n];
        int left = 0, right = n - 1;
        int top = 0, bottom = n - 1;
        int count = 1;
        while(left <=right && top <= bottom){
            // fill the top
            for(int col = left; col <= right; col++){
                result[top][col] = count++;
            }
            top++;
            // fill the right
            for(int row = top; row <= bottom; row++){
                result[row][right] = count++;
            }
            right--;
            // fill the bottom
            for(int col = right; col >= left; col--){
                result[bottom][col] = count++;
            }
            bottom--;
            // fill the left
            for(int row = bottom; row >= top; row--){
                result[row][left] = count++;
            }
            left++;
        }
        return result;
    }
}

Additional