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 ]]

这道题的思路和54. Spiral Matrix的思路十分相似，先初始化一个n*n的二维数组，然后通过colBegin,colEnd,rowBegin,rowEnd四个值来实现旋转顺序赋值。
利用vector实现二维数组的方法：resize()函数
使用vector构造二维数组

class Solution {public:    vector<vector<int>> generateMatrix(int n) {        vector<vector<int>> ans(n);        for (int i = 0; i < n; i++) {            ans[i].resize(n);        }        if (n == 0) return ans;        int rowBegin = 0, rowEnd = n - 1, colBegin = 0, colEnd = n - 1;        int num = 1;        while (rowBegin <= rowEnd && colBegin <= colEnd) {            for (int i = colBegin; i <= colEnd; i++) {                ans[rowBegin][i] = num;                num++;            }            rowBegin++;            for (int i = rowBegin; i <= rowEnd; i++) {                ans[i][colEnd] = num;                num++;            }            colEnd--;            if (rowBegin <= rowEnd) {                for (int i = colEnd; i >= colBegin; i--) {                    ans[rowEnd][i] = num;                    num++;                }                rowEnd--;            }            if (colBegin <= colEnd) {                for (int i = rowEnd; i >= rowBegin; i--) {                    ans[i][colBegin] = num;                    num++;                }                colBegin++;            }        }        return ans;    }};