class Solution {
public:
	vector<int> findDiagonalOrder(vector<vector<int>>& mat) {
		int m = mat.size(), n = mat[0].size();
		vector<int> res;
		res.reserve(m * n); // 预分配空间
		for (int k = 0;k < m + n - 1;k++) {
			int min_j = max(k - m + 1, 0);
			int max_j = min(k, n - 1);
			
			if (k % 2 == 0) {
				//偶数从左到右
				for (int j = min_j;j <= max_j;j++) {
					res.push_back(mat[k - j][j]);
				}
			}
			else {
				//奇数从右到左
				for (int j = max_j;j >= min_j;j--) {
					res.push_back(mat[k - j][j]);
				}
			}
		}
		return res;
	}
};