[LeetCode] Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

[ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ]]

You should return [1,2,3,6,9,8,7,4,5].

class Solution {public:    vector<int> spiralOrder(vector<vector<int> > &matrix) {        vector<int> result;        if(matrix.size() == 0)            return result;        int x1 = 0,y1 = 0,x2 = matrix.size() - 1,y2 = matrix[0].size() - 1;        while(x1 <= x2 && y1 <= y2){            for(int i = y1;i <= y2;i ++) result.push_back(matrix[x1][i]);            for(int i = x1 + 1;i <= x2;i ++) result.push_back(matrix[i][y2]);            if(x1 != x2)                for(int i = y2 - 1;i >= y1;i --) result.push_back(matrix[x2][i]);            if(y1 != y2)                for(int i = x2 - 1;i > x1;i --) result.push_back(matrix[i][y1]);            x1 ++;x2 --;y1 ++;y2 --;        }        return result;    }};