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MEDIUM

Spiral Matrix II

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n^2 in spiral order.

Example

Input:

n = 3

Output:

[[1,2,3],[8,9,4],[7,6,5]]

Explanation:
The matrix is filled in spiral order.

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Constraints

• 1 ≤ n ≤ 20

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Solution: Simulation

• Time Complexity: O(n^2)
• Space Complexity: O(n^2)

C++

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