Description
Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.
For example,
Given n = 3
,
You should return the following matrix:
[
[ 1, 2, 3 ],
[ 8, 9, 4 ],
[ 7, 6, 5 ]
]
Solution
Iteration, time O(n ^ 2), space O(1)
class Solution {
public int[][] generateMatrix(int n) {
if (n < 1) {
return new int[0][0];
}
int[][] matrix = new int[n][n];
int count = 0;
for (int i = 0; i < (n + 1) / 2; ++i) {
if (i == n - i - 1) {
matrix[i][i] = ++count; // single item in the center
break;
}
for (int j = i; j < n - i - 1; ++j) {
matrix[i][j] = ++count;
}
for (int j = i; j < n - i - 1; ++j) {
matrix[j][n - i - 1] = ++count;
}
for (int j = n - i - 1; j > i; --j) {
matrix[n - i - 1][j] = ++count;
}
for (int j = n - i - 1; j > i; --j) {
matrix[j][i] = ++count;
}
}
return matrix;
}
}