0059-spiral-matrix-ii.cpp

class Solution {
 public:
  vector<vector<int>> generateMatrix(int n) {
    vector<vector<int>> result(n, vector<int>(n));
    int cnt = 1;
    for (int layer = 0; layer < (n + 1) / 2; layer++) {
      // direction 1 - traverse from left to right
      for (int ptr = layer; ptr < n - layer; ptr++) {
        result[layer][ptr] = cnt++;
      }
      // direction 2 - traverse from top to bottom
      for (int ptr = layer + 1; ptr < n - layer; ptr++) {
        result[ptr][n - layer - 1] = cnt++;
      }
      // direction 3 - traverse from right to left
      for (int ptr = n - layer - 2; ptr >= layer; ptr--) {
        result[n - layer - 1][ptr] = cnt++;
      }
      // direction 4 - traverse from bottom to top
      for (int ptr = n - layer - 2; ptr > layer; ptr--) {
        result[ptr][layer] = cnt++;
      }
    }
    return result;
  }
};