P9910 题目链接。

使用了 set<int> suf_min 维护，通过玩样例发现当其中值为 $[3, 4, 5]$ 时，一个迭代器 it 指向 $3$，但是 ++it 后，迭代器没有指向 $4$ 反而指向了 $5$，又进行一次 --it 发现才指向 $4$，请问代码哪里有问题，或者猜测是哪里访问越界了，但是在线IDE运行却没有报错？

贴上测试代码以及相关内容。

#include <cstdio>
#include <set>
#include <iostream>
using namespace std;

int n, q, a[100010], inv[100010];
long long ans;
set<int> suf_min;

struct Segment_Tree {
#define lson (idx << 1    )
#define rson (idx << 1 | 1)
	struct node {
		int l, r, minn;
	} tree[100010 << 2];
	void pushup(int idx) {
		tree[idx].minn = min(tree[lson].minn, tree[rson].minn);
	}
	void build(int idx, int l, int r) {
		tree[idx] = {l, r, n + 1};

		if (l == r) return tree[idx].minn = a[l], void();

		int mid = (l + r) >> 1;
		build(lson, l, mid), build(rson, mid + 1, r), pushup(idx);
	} int query(int idx, int l, int r) {
		if (tree[idx].l > r || tree[idx].r < l) return n + 1;

		if (l <= tree[idx].l && tree[idx].r <= r) return tree[idx].minn;

		return min(query(lson, l, r), query(rson, l, r));
	} void modify(int idx, int p, int v) {
		if (tree[idx].l > p || tree[idx].r < p) return;

		if (tree[idx].l == tree[idx].r) return tree[idx].minn = v, void();

		modify(lson, p, v), modify(rson, p, v), pushup(idx);
	}
#undef lson
#undef rson
} yzh;

struct Bit_Tree {
	constexpr inline int lowbit(int x) {
		return x & -x;
	}
	int tree[100010];
	void modify(int p, int v) {
		for (int i = p; i <= n; i += lowbit(i)) tree[i] += v;
	}
	int query(int p) {
		int res = 0;

		for (int i = p; i; i ^= lowbit(i)) res += tree[i];

		return res;
	}
	int query(int l, int r) {
		return query(r) - query(l - 1);
	}
} marked_value, marked_idx, removed_value, removed_idx;

void mark(int i) {
	suf_min.insert(i), marked_value.modify(a[i], 1), marked_idx.modify(i, 1);
	ans += (n - a[i] - removed_value.query(1, a[i])) - (n - i - removed_idx.query(1, i));
}

void remove(int i) {
	ans -= (n - a[i] - removed_value.query(1, a[i])) - (n - i - removed_idx.query(1, i));
	marked_value.modify(a[i], -1), marked_idx.modify(i, -1);
	int L = n, R = 0;
	
	// -------------------------------------------------------------------------------------
	
	auto it = suf_min.find(i);

	cout << "Suf_set info ->";

	for (auto x : suf_min) cout << x << ' ';

	cout << endl;

	cout << *it << endl;

	++it;

	cout << *it << endl;


	--it;

	cout << *it << endl;

	--it;


	cout << *it << endl;
	
	// -------------------------------------------------------------------------------------

	exit(0);

	if (it != suf_min.end()) R = *it;
	else R = n + 1;

	--it;

	if (it != suf_min.begin()) L = *--it;
	else L = 1;


	suf_min.erase(suf_min.find(a[i]));

	while (L < R) {
		int j = yzh.query(1, L, R - 1);

		if (j == n + 1) break;

		j = inv[j];

		if (a[j] < (R == n + 1 ? n + 1 : a[R])) mark(j);

		L = j + 1;
	}
}

signed main() {
	scanf("%d%d", &n, &q);

	for (int i = 1; i <= n; ++i) scanf("%d", &a[i]), inv[a[i]] = i;

	yzh.build(1, 1, n), ans = n;
	int minn = n + 1;

	for (int i = n; i >= 1; --i) if (a[i] < minn) minn = a[i], mark(i);

	// printf("%lld ", ans);

	for (int i = 1; i <= q; ++i) {
		int x;
		scanf("%d", &x), yzh.modify(1, x, n + 1);

		if (suf_min.count(a[x])) remove(x);

		ans += marked_value.query(a[x], n) - 1 - marked_idx.query(x, n) - 1 - 1;
		removed_value.modify(a[x], 1), removed_idx.modify(x, 1);
		// printf("%lld ", ans);
	}

	return 0;
}

输入

5 2
3 4 1 2 5
3 2

输出

Suf_set info ->3 4 5 
3
5
4
3