Spiral Matrix

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题目：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 ]]

思路：

四条线路法，定义up，right，down，left四个变量，先走最上面，再走最右边，再走最下面，再走最左边，条件结束标志为左等于右，上等于下。

代码：

class Solution {public:    vector<int> spiralOrder(vector<vector<int>>& matrix) {        vector<int>res;                if(matrix.empty() || matrix[0].empty()){            return res;        }        int m=matrix.size();        int n=matrix[0].size();        int left=0,right=n-1;        int up=0,down=m-1;                while(up<=down&&left<=right){            for(int i=left;i<=right;i++){                res.push_back(matrix[up][i]);            }                        for(int i=up+1;i<=down;i++){                res.push_back(matrix[i][right]);            }                        if(up==down||left==right){                    break;            }                for(int i=right-1;i>=left;i--){                    res.push_back(matrix[down][i]);                }                            for(int i=down-1;i>=up+1;i--){                    res.push_back(matrix[i][left]);                }            up++;            right--;            down--;            left++;        }                return res;    }};

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