Spiral Matrix II

/*
LeetCode 59:
Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

Solution:
Language: C++
Tag:
*/

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        if (n <= 0) {
            return vector<vector<int>>();
        }
        // 1. O(mn) time + O(1) space
        vector<vector<int>> res(n, vector<int>(n, 1));
        int trans[4][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}}; // R D L U
        int left = 0, right = n, up = 0, down = n;
        int val = 1, dir = 0;
        pair<int, int> pos({0, 0});
        while (val < n * n) {
            int row = pos.first + trans[dir][0], col = pos.second + trans[dir][1];
            if (!inbound(row, col, left, right, up, down)) {
                if (dir == 0) {
                    ++up;
                } else if (dir == 1) {
                    --right;
                } else if (dir == 2) {
                    --down;
                } else {
                    ++left;
                }
                dir = (dir + 1) % 4;
                continue;
            }
            res[row][col] = ++val;
            pos.first = row;
            pos.second = col;
        }
        return res;
    }
    
    bool inbound(int row, int col, int left, int right, int up, int down) {
        if (row >= up && row < down && col >= left && col < right) {
            return true;
        }
        return false;
    }
};