0440_K-th_Smallest_in_Lexicographical_Order.cpp

// ███████╗  █████╗  ███╗   ██╗ ██████╗   █████╗  ██████╗  ██████╗  ██╗  ██╗
// ██╔════╝ ██╔══██╗ ████╗  ██║ ██╔══██╗ ██╔══██╗ ██╔══██╗ ██╔══██╗ ██║  ██║
// ███████╗ ███████║ ██╔██╗ ██║ ██║  ██║ ███████║ ██████╔╝ ██████╔╝ ███████║
// ╚════██║ ██╔══██║ ██║╚██╗██║ ██║  ██║ ██╔══██║ ██╔═██╗  ██╔══██╗ ██╔══██║
// ███████║ ██║  ██║ ██║ ╚████║ ██████╔╝ ██║  ██║ ██║  ██╗ ██████╔╝ ██║  ██║
// ╚══════╝ ╚═╝  ╚═╝ ╚═╝  ╚═══╝ ╚═════╝  ╚═╝  ╚═╝ ╚═╝  ╚═╝ ╚═════╝  ╚═╝  ╚═╝
#pragma GCC optimize("Ofast", "inline", "ffast-math", "unroll-loops","no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native", "f16c")
auto init = []() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    return 'c';
}();

class Solution {
public:
    int n;
    int findKthNumber(int n, int k) {
        this->n = n;
        --k;
        long long curr = 1;
        while (k) {
            int cnt = count(curr);
            if (k >= cnt) {
                k -= cnt;
                ++curr;
            } else {
                --k;
                curr *= 10;
            }
        }
        return (int) curr;
    }

    int count(long long curr) {
        long long next = curr + 1;
        int cnt = 0;
        while (curr <= n) {
            cnt += min(n - curr + 1, next - curr);
            next *= 10;
            curr *= 10;
        }
        return cnt;
    }
};