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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;}
};