class Solution {
public:
    vector<int> spiralOrder(vector<vector<int> > &matrix) {
        vector<int> res;
        int m = matrix.size();
        int n = matrix[0].size();
        if(m==0 || n==0) return res;//数组为空
        
        int left = 0,right = n-1;
        int top = 0,btm = m-1;
        while(left<=right && top<=btm){
            //从左往右遍历
            for(int i=left;i<=right;i++){
                res.push_back(matrix[top][i]);
            }
            //从上往下遍历
            for(int j=top+1;j<=btm;j++){
                res.push_back(matrix[j][right]);
            }
            //从右往左遍历
            if(top < btm){//case:[[1,2,3,4],[5,6,7,8],[9,10,11,12]],不加if判断时输出[1,2,3,4,8,12,11,10,9,5,6,7,6]，因为满足for的条件，执行循环体后，结果中多了个6
                for(int k=right-1;k>=left;k--){
                    res.push_back(matrix[btm][k]);
                }
            }
            //从下往上遍历
            if(left < right){//case:[[1,2,3],[5,6,7],[9,10,11],[13,14,15],[17,18,19]],不加if判断时输出[1,2,3,7,11,15,19,18,17,13,9,5,6,10,14,10,14],因为满足for的条件，多输出10，14
                for(int m=btm-1;m>top;m--){
                    res.push_back(matrix[m][left]);
                }
            }
            
            top++;
            btm--;
            left++;
            right--;
        }
        return res;
    }
};

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