L59. Spiral Matrix II

Problem:

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

Example:

Input: 3
Output:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

Solution:

• temp = 1 since we start at 1 instead of 0;

public int[][] generateMatrix(int n) {
    if(n <= 0) return null;
    final int[][] result = new int[n][n];
    int rowStart = 0, colStart = 0;
    int rowEnd = n-1, colEnd = n-1;
    int temp = 1;
        
    while(rowStart <= rowEnd && colStart <= colEnd){
        //Traverse right
        for(int i = colStart; i <= colEnd; i++){
            result[rowStart][i] = temp++;
        }
        rowStart++;
        //Traverse down
        for(int i = rowStart; i <= rowEnd; i++){
            result[i][colEnd] = temp++;
        }
        colEnd--;
        //Taverse left
        if(rowEnd >= rowStart){
            for(int i = colEnd; i >= colStart; i--){
            result[rowEnd][i] = temp++;
        }
        rowEnd--;
        }
        //up
        if(colEnd >= colStart){
            for(int i = rowEnd; i >= rowStart; i--){
            result[i][colStart] = temp++;
        }
                colStart++;
            }
    }
    return result;
}