算法1：（利用了对角线的性质）

#include <iostream>
#include <vector>
#include <map>

using namespace std;

int n, t;

int main(void) {
  scanf("%d", &n);
  map<int, vector<int>> m;
  for (int y = 0; y < n; ++y)
    for (int x = 0; x < n; ++x) {
      scanf("%d", &t);
      m[x + y].emplace_back(t);
    }

  for (auto&& [id, vals] : m)
    if (id & 1) for (const auto& x : vals) printf("%d ", x);
    else for (auto it = vals.rbegin(); it != vals.rend(); ++it) printf("%d ", *it);

  return 0;
}

算法2（使用图遍历的方法）

欠债还钱 天津第一

#include <iostream>
#include <vector>

using namespace std;
int n;

void findDiagonalOrder(vector<vector<int>>& matrix) {
  int x = 0, y = 0, d = 1, cnt = 0;
  while (cnt++ < n * n) {
    printf("%d ", matrix[y][x]);
    const int nx = x + d, ny = y - d;
    if (nx < 0 || ny < 0 || nx == n || ny == n) {
      if (d == 1) {
        if (nx == n) ++y;
        else ++x;
      } else {
        if (ny == n) ++x;
        else ++y;
      }
      d = -d;
      continue;
    }
    x = nx;
    y = ny;
  }
}

int main(void) {
  scanf("%d", &n);
  vector<vector<int>> matrix(n, vector<int>(n));
  for (int y = 0; y < n; ++y)
    for (int x = 0; x < n; ++x) cin >> matrix[y][x];

  findDiagonalOrder(matrix);
}