数组——螺旋矩阵II

题目顺序：代码随想录算法公开课，b站上有相应视频讲解

一、题目

59. Spiral Matrix II

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.

Example 1:

Input: n = 3
Output: [[1,2,3],[8,9,4],[7,6,5]]
Example 2:

Input: n = 1
Output: [[1]]

Constraints:

1 <= n <= 20

二、题解

循环不变量

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> res(n,vector<int>(n,0));
        int startX = 0,startY = 0,offset = 1,count = 1;
        int i,j;
        int loops = n / 2;
        while(loops--){
            i = startX,j = startY;
            for(j = startY;j < n - offset;j++) res[i][j] = count++;
            for(i = startX;i < n - offset;i++) res[i][j] = count++;
            for(;j > startY;j--) res[i][j] = count++;
            for(;i > startX;i--) res[i][j] = count++;
            startX++;
            startY++;
            offset++;
        }
        if(n % 2 == 1) res[n/2][n/2] = count;
        return res;
    }
};