输入：$A=a_1,a_2,...,a_m(0<=a_i<=9,1<=m<=10);n$

输出：构造小于n的最大数（可重复使用$a_i$）

思路

$从n的最高位开始遍历，考虑当前位置可取的最大a_i，使用stack来存储每一位选择的a$

#include <bits/stdc++.h>using namespace std;
using ll = long long;const int mod = 1e9 + 7;void solve() {int A[] = {9, 8};int n = 9;n --;map<int, int> mp;for(auto x: A) mp[x] = 1;int a[10], cnt = 0;while(n) {a[cnt ++] = n % 10;n /= 10;}function<int(int)> fin = [&](int x) -> int {for(int i = x; i >= 0; -- i) {if(mp[i]) return i;}return -1;};bool f = false;stack<int> st;if(fin(a[cnt - 1]) == -1) {for(int i = cnt - 2; i >= 0; -- i) {st.push(fin(9));}} else {for(int i = cnt - 1; i >= 0; -- i) {if(f) {st.push(fin(9));continue;}int x = fin(a[i]);if(x == -1) {while(!st.empty()) {int _x = st.top();x = fin(_x - 1);st.pop();i ++;if(x == -1) continue;st.push(x);f = true;break;}} else {if(x != a[i]) f = true;st.push(x);}}}string ans = "";while(!st.empty()) {ans = char(st.top() + '0') + ans;st.pop();}cout << ans << '\n';
}signed main() {ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#ifdef ACM_LOCALfreopen("in", "r", stdin);freopen("out", "w", stdout);signed test_index_for_debug = 1;char acm_local_for_debug = 0;do {if (acm_local_for_debug == '$') exit(0);if (test_index_for_debug > 20)throw runtime_error("Check the stdin!!!");auto start_clock_for_debug = clock();solve();auto end_clock_for_debug = clock();cout << "Test " << test_index_for_debug << " successful!" << endl;cerr << "Test " << test_index_for_debug++ << " Run Time: "<< double(end_clock_for_debug - start_clock_for_debug) / CLOCKS_PER_SEC << "s" << endl;cout << "--------------------------------------------------" << endl;} while (cin >> acm_local_for_debug && cin.putback(acm_local_for_debug));
#elsesolve();
#endifreturn 0;
}