Leetcode Spiral Matrix II

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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 ]]

class Solution {public:    vector<vector<int> > generateMatrix(int n) {        vector<vector<int> > res(n,vector<int>(n,0));        int dx[] = {0,1,0,-1};        int dy[] = {1,0,-1,0};        int cnt = 1,step = 0,x = 0 , y = 0;        while(cnt <= n*n){            step%=4;            res[x][y] = cnt++;            if(x+dx[step] >=n || x+dx[step] < 0 || y+dy[step] >=n || y+dy[step] < 0 || res[x+dx[step]][y+dy[step]]) step++;            x+=dx[step%4];y+=dy[step%4];        }        return res;    }};