nth natural number.cpp

/*
///////////////////////////////////////////
//Question/Info

Nth Natural Number
Hard Accuracy: 41.17% Submissions: 18076 Points: 8
Given a positive integer N. You have to find Nth natural number after removing all the numbers containing digit 9 .


Example 1:

Input:
N = 8
Output:
8
Explanation:
After removing natural numbers which contains
digit 9, first 8 numbers are 1,2,3,4,5,6,7,8
and 8th number is 8.
Example 2:

Input:
N = 9
Output:
10
Explanation:
After removing natural numbers which contains
digit 9, first 9 numbers are 1,2,3,4,5,6,7,8,10
and 9th number is 10.


Your Task:
You don't need to read input or print anything. Complete the function findNth() which accepts an integer N as input parameter and return the Nth number after removing all the numbers containing digit 9.


Expected Time Complexity: O(logN)
Expected Auxiliary Space: O(1)


Constraints:
1 ≤ N ≤ 1012

author: srj_v
///////////////////////////////////////////
*/

#include <bits/stdc++.h>
using namespace std;

// #define int 		   long long int
#define sbit(x) 	   __builtin_popcount(x)
#define pb(x) 		   push_back(x)
#define mp(x,y) 	   make_pair(x,y)
#define eb(x) 		   emplace_back(x)
#define ct(x) 		   cout << x << "\n";
#define ct2(x,y) 	   cout << x << " " << y << "\n";
#define tc(x) 		   cout << x << " ";
#define tc2(x,y) 	   cout << x << " " << y << " ";
#define forn(i,n) 	   for(int i = 0; i < (int)(n); ++i)
#define forx(i,x,n)    for(int i = x; i < (int)(n); ++i)
#define nfor(i,n)      for(int i = n-1; i >= 0; --i)
#define all(v) 		   v.begin(),v.end()
#define fsp(x,y)       fixed << setprecision(y) << x
#define PI 			   3.1415926535897932384626433832795
#define MOD 		   1000000007 // (1e9+7)
#define pii 		   pair<int,int>
#define pis 		   pair<int,string>
#define vi  		   vector<int>
#define vii 		   vector<pii>
#define mii 		   map<int,int>
#define p_q            priority_queue // priority_queue<int> (&) priority_queue< int,vi,greater<int> >
#define _IOS      	   ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

typedef long double ld;
typedef long long int lli;

#pragma GCC optimize("Ofast")

void c_p_c()
{
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
}


int32_t main() {
///////////
	c_p_c();
///////////
	_IOS
//////////

	// code

	/*
	int t ; cin >> t; while(t--){}
	*/



	class Solution {
	public:
		long long findNth(long long N)
		{

			long long ans = 0;

			long long x = 1;

			while (N > 0) {

				ans += (x * (N % 9));

				N = N / 9;

				x = x * 10;
			}

			return ans ;
		}
	};




// cerr << "time: " << clock() << " ms" << '\n';
	return 0;
}