1169C - Increasing by Modulo - CodeForces Solution

#include <bits/stdc++.h>

using namespace std;

int main()
{
	int n, m; scanf("%d %d", &n, &m);
	vector<int> a(n + 1);
	for (int i = 1; i <= n; i++)
		scanf("%d", &a[i]);
	
	auto check = [&] (int val)
	{
		auto b = a;
		
		for (int i = 1; i <= n; i++)
		{
			int need = (b[i - 1] - b[i] + m) % m;
			if (need <= val) b[i] = b[i - 1];
			else if (b[i] < b[i - 1]) return false;
 		}
 		return true;
	};
	
	int l = 0, r = m;
	while (l < r)
	{
		int mid = l + r >> 1;
		if (check(mid)) r = mid;
		else l = mid + 1;
	}
	printf("%d\n", r);
	
	return 0;
}