【数据结构】Decreasing String—CF1886C

Decreasing String—CF1886C

代码我现在还不是很理解，群友说是单调栈。

$Code$

#include 
#define int long long
#define sz(a) ((int)a.size())
#define all(a) a.begin(), a.end()
using namespace std;
using PII = pair<int, int>;
using i128 = __int128;
const int N = 2e5 + 10;

int n;

void solve() {
	string s; cin >> s;
	n = sz(s);
	s.insert(s.begin(), ' ');
	int pos; cin >> pos;
	
	int cut = 0; // 还剩几个字符需要删除
	int idx = -1; // 在t字符串中的下标
	for (int i = n; i >= 1; i --) {
		if (pos > i) {
			pos -= i;
		} else {
			cut = n - i;
			idx = pos;
			break;
		}
	}
	
	string t;
	for (int i = 1; i <= n; i ++) {
		while (not t.empty() && cut && s[i] < t.back()) {
			t.pop_back();
			cut --;
		}
		t += s[i];
	}
	
	cout << t[idx - 1];
}

signed main() {
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	int T = 1;
	cin >> T; cin.get();
	while (T --) solve();
	return 0;
}