59. 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 ]
]
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蛇形填数这个题目不管在《算法竞赛入门经典》，还是《剑指OFFER》 都看过。
不过这还是第一次动手去写吧。思路比较清楚，就是要注意循环的时候，容易越界访问。

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int> > ans (n,vector<int>(n,0));
        if(n<1)return ans;
        int tot=1;
        int i=0,j=0,k=1;
        ans[i][j]=k++;
        while(tot++<n*n){
            //这里的j+1是关键，用j<n的话，下一个循环的时候就会约界访问数组
            while(j+1<n&&ans[i][j+1]==0)ans[i][++j]=k++;
            while(i+1<n&&ans[i+1][j]==0)ans[++i][j]=k++;
            while(j-1>=0&&ans[i][j-1]==0)ans[i][--j]=k++;
            while(i-1>=0&&ans[i-1][j]==0)ans[--i][j]=k++;
        }
        return ans;
    }
};