59. Spiral Matrix II

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 ]
]

题意就是做一个n*n的回环。
我的思路是定义x，y记录位置，index记录数值。一圈一圈向内填充。最外层循环4次 每次循环n-1次，最后x=0，y=0。x++，y++后进入更内一层，4*(n-3)的循环，以此类推。

class Solution {
public int[][] generateMatrix(int n) {
int num[][] = new int

;
int x = 0;
int y = 0;
int index = 1;
for(int i = 1;i<=n/2;i++){
for(int j = 0;j<n+1-i*2;j++){
num[x][y] = index;
index++;
y++;
}
for(int j = 0;j<n+1-i*2;j++){
num[x][y] = index;
index++;
x++;
}
for(int j = 0;j<n+1-i*2;j++){
num[x][y] = index;
index++;
y--;
}
for(int j = 0;j<n+1-i*2;j++){
num[x][y] = index;
index++;
x--;
}
x++;
y++;
}
if(n%2 == 1)
num[n/2][n/2] = n*n;
return num;
}
}