59. Spiral Matrix II

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 ]
]

vector<vector<int>> generateMatrix(int n) { // time: O(n^2); space: O(n^2)
    vector<vector<int> > res(n, vector<int>(n, 0));
    int val = 1, i = 0;
    while (val <= n * n) {
        int j = i;
        // horizontal, left->right
        while (j < n - i) res[i][j++] = val++;
        j = i + 1;
        // vertical, top->bototm
        while (j < n - i) res[j++][n - i - 1] = val++;
        j = n - i - 2;
        // horizontal, right->left
        while (j > i) res[n - i - 1][j--] = val++;
        j = n - i - 1;
        // vertical, bottom->top
        while (j > i) res[j--][i] = val++;
        ++i; // go to the next loop
    }
    return res;
}