leetcode 54. Spiral Matrix

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

Example 1:

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

Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> res;
        int m = matrix.size();
        if (m == 0)
            return res;
        int n = matrix[0].size();
        int cnt = 0;
        res.resize(m * n);
        int start_row = 0, start_col = 0, end_row = m - 1, end_col = n - 1;
        while (cnt < m * n) {
            for (int j = start_col; cnt < m * n && j <= end_col; j++) {
                res[cnt++] = matrix[start_row][j];
            }
            for (int i = start_row + 1; cnt < m * n && i <= end_row; i++) {
                res[cnt++] = matrix[i][end_col];
            }
            for (int j = end_col - 1; cnt < m * n && j >= start_col; j--) {
                res[cnt++] = matrix[end_row][j];
            }
            for (int i = end_row - 1; cnt < m * n && i > start_row; i--) {
                res[cnt++] = matrix[i][start_col];
            }
            start_row++;
            start_col++;
            end_row--;
            end_col--;
        }
        return res;
    }
};