386. Lexicographical Numbers

Problem:

Given an integer n, return 1 - n in lexicographical order.

For example, given 13, return: [1,10,11,12,13,2,3,4,5,6,7,8,9].

Please optimize your algorithm to use less time and space. The input size may be as large as 5,000,000.

思路：

Solution (C++):

vector lexicalOrder(int n) {
    vector res;
    helper(1, n, res);
    return res;
}
void helper(int target, int n, vector& res) {
    if (target > n)  return;
    res.push_back(target);
    helper(target*10, n, res);
    if (target % 10 != 9)  helper(target+1, n, res);
}

性能：

Runtime: 68 ms  Memory Usage: 9.8 MB

思路：

Solution (C++):

vector lexicalOrder(int n) {
    vector res(n, 0);
    int cur = 1;
    for (int i = 0; i < n; ++i) {
        res[i] = cur;
        if (cur * 10 <= n)  cur *= 10;
        else {
            if (cur >= n)  cur /= 10;
            ++cur;
            while (cur%10 == 0)  cur /= 10;
        }
    }
    return res;
}

性能：

Runtime: 60 ms  Memory Usage: 8.7 MB