1291. Sequential Digits

<<--- The first solution is without using Recursion--->>

class Solution {
public:
    vector<int> sequentialDigits(int low, int high) {
        vector<int>v;
        int s = 0, i = 1, digt = digit(low), n;
        while (s < high) {
            s = 0;n = digt;
            int j = i;
            while (n--) s = s * 10 + j++;
            if (s <= high && s >= low)   v.push_back(s);
            if (s % 10 == 9) {
                digt++;
                i = 0;
            }
            i++;
    }
    return v;
}
int digit(int nums) {
    int count = 0;
    while (nums) {  nums /= 10; count++;  }
    return count;
}
};

<<--- The second solution is to use Recursion--->>

class Solution {
public:
vector<int>v;
    vector<int> sequentialDigits(int low, int high) {
        for (int i = 1; i <= 9; i++) 
            dfs(low, high, i, 0);
        sort(v.begin(), v.end());
        return v;
}
void dfs(int low, int high, int i, int num) {
        if (num <= high && num >= low)  v.push_back(num);
        if (i > 9 || num > high) return;
        dfs(low, high, i + 1, num * 10 + i);
}
};