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 ]
]

AC code:

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> ans(n, vector<int>(n, 0));
        int top = 0, bottom = n-1;
        int left = 0, right = n-1;
        int num = 1;
        while (top <= bottom && left <= right) {
            // left -> right
            for (int i = left; i <= right; ++i) {
                ans[top][i] = num++;
            }
            top++;
            // top -> bottom
            for (int i = top; i <= bottom; ++i) {
                ans[i][right] = num++;
            }
            right--;
            // right -> left
            for (int i = right; i >= left; --i) {
                ans[bottom][i] = num++;
            }
            bottom--;
            // bottom -> top
            for (int i = bottom; i >= top; --i) {
                ans[i][left] = num++;
            }
            left++;
        }
        return ans;
    }
};

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Spiral Matrix II.