[leetcode] Spiral Matrix II

From : https://leetcode.com/problems/spiral-matrix-ii/

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) {        if(n < 0 ) n=(~n)+1;        vector<vector<int>> ans(n, vector<int>(n));        int u=0, d=n-1, l=0, r=n-1, v=1, nn=n*n;        while(v<=nn) {            for(int i=l; i<=r; i++) ans[u][i] = v++;            if(v>nn) break;            u++;            for(int i=u; i<=d; i++) ans[i][r] = v++;            if(v>nn) break;            r--;            for(int i=r; i>=l; i--) ans[d][i] = v++;            if(v>nn) break;            d--;            for(int i=d; i>=u; i--) ans[i][l] = v++;            if(v>nn) break;            l++;        }        return ans;    }};

public class Solution {    public int[][] generateMatrix(int n) {        int[][] m = new int[n][n];        int l=0, t=0, r=n-1, b=n-1;        int u = 0;        while(l<=r && t<=b) {            for(int i=l; i<=r; ++i) {                m[t][i] = ++u;            }            ++t;            for(int i=t; i<=b; ++i) {                m[i][r] = ++u;            }            --r;            for(int i=r; i>=l; --i) {                m[b][i] = ++u;            }            --b;            for(int i=b; i>=t; --i) {                m[i][l] = ++u;            }            ++l;        }        return m;    }}