算法分析与设计课程——LeetCode刷题之 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 ]
]

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

答案：

class Solution {
public:
   vector<int> spiralOrder(vector<vector<int>>& matrix) {
    vector<vector<int> > dirs{{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
    vector<int> res;
    int nr = matrix.size();     if (nr == 0) return res;
    int nc = matrix[0].size();  if (nc == 0) return res;
    
    vector<int> nSteps{nc, nr-1};
    
    int iDir = 0;   // index of direction.
    int ir = 0, ic = -1;    // initial position
    while (nSteps[iDir%2]) {
        for (int i = 0; i < nSteps[iDir%2]; ++i) {
            ir += dirs[iDir][0]; ic += dirs[iDir][1];
            res.push_back(matrix[ir][ic]);
        }
        nSteps[iDir%2]--;
        iDir = (iDir + 1) % 4;
    }
    return res;
}
};