LeetCode 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 ]]

answer:

class Solution {public:    vector<vector<int>> generateMatrix(int n) {                vector<vector<int>> result;        if(n == 0) return result;        else if( n < 0) n = -n;        //result.reserve(n);        vector<int> temp;        for(int i = 0; i < n; i ++)            temp.push_back(0);        for(int i = 0; i < n; i ++)            //result[i].reserve(n);            result.push_back(temp);        int left = 0, right = n - 1, length = n;        int index = 0;        while(length > 0){            for(int i = 0; i < length; i ++){                result[left][left + i] = ++ index;            }            cout << index << endl;            for(int i = 1; i < length; i ++){                result[left + i][right] = ++ index;            }            cout << index << endl;            for(int i = 1; i < length; i ++){                result[right][right - i] = ++ index;            }            cout << index << endl;            for(int i = 1; i < length - 1; i ++){                result[right - i][left] = ++ index;            }            cout << index << endl;            length -= 2;            left ++;            right --;        }        return result;    }};