59. Spiral Matrix II

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problem：

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 ]]

按顺序编写即可，以每行的(i,i)为起点

class Solution {public:    vector<vector<int>> generateMatrix(int n) {        vector<vector<int>> result(n, vector<int>(n));        if(n == 0)            return result;        int layer = (n + 1) / 2;        int initial_num = 1;        for(int i=0; i<layer; i++)        {            //row i            int first = initial_num;            for(int j=i; j<n-i; j++)            {                                result[i][j] = first + j - i;            }                        //col n-i-1            first = result[i][n-i-1];            for(int j=i+1; j<n-i; j++)            {                result[j][n-i-1] = first + j - i;            }                        //row n-i-1            first = result[n-i-1][n-i-1];            for(int j=n-i-1-1; j>=i; j--)            {                result[n-i-1][j] = first + n - i - 1 - j;            }                        //col i            first = result[n-1-i][i];            for(int j=n-i-1-1; j>i; j--)            {                result[j][i] = first + n - i - 1 - j;            }            initial_num += (n-i*2-1)*4;        }        return result;    }};

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