59. Spiral Matrix II

class Solution {
public:
vector<vector<int>> generateMatrix(int n) {
vector<vector<int>> res(n, vector<int>(n));

int cycle = (n+1)/2, num=1;
for (int i=0; i<cycle; i++) { // i代表第几个环
// 每个环的左上角起点是matrix[i][i],下面顺时针依次填充环的四条边
for (int j=i; j<n-i; j++) {
res[i][j]=num++;
}
for (int j=i+1; j<n-i; j++) {
res[j][n-i-1] = num++;
}

for (int j=n-i-2; j>=i; j--) {
res[n-i-1][j] = num++;
}
for (int j=n-i-2; j>i; j--) {
res[j][i] = num++;
}
}
return res;
}
};