Submission #55788066

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
 
#define IOS ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define int long long
#define endl "\n"

const int N = 105;

int n;

int getNumPalindromes(int digits) {
	if(digits == 1)
		return 10;
	int modifiable = (digits + 1) / 2;
	int firstDigit = 9; // 1 to 9
	int otherDigits = 1;
	for(int i = 1; i <= modifiable - 1; i++) {
		otherDigits *= 10; // 0 to 9
	}
	return firstDigit * otherDigits;
}

void nthPalindromeOfKdigits(int k) { // 2: 1, 3: 10, 4:10
	if(k == 1) {
		cout << n - 1;
		return;
	}
	int firstHalf = (k - 1) / 2;
	int value = 1;
	for(int i = 1; i <= firstHalf; i++) {
		value *= 10;
	}
	value += (n - 1);
	cout << value;
	if(k % 2)
		value /= 10;
	while(value) {
		cout << value % 10;
		value /= 10;
	}
}

int32_t main() {
	IOS;
	cin >> n;
	int k = 1; // Number of digits
	while(getNumPalindromes(k) < n) {
		n -= getNumPalindromes(k);
		k++;
	}
	nthPalindromeOfKdigits(k);
	return 0;
}

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