洛谷P1155 双栈排序(贪心)

// luogu-judger-enable-o2
#include<bits/stdc++.h>
#define NO {puts("0"); exit(0);}
#define pb(x) push_back(x) 
using namespace std;
const int MAXN = 1001;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int N, a[MAXN], s1[MAXN], tp1, s2[MAXN], tp2;
vector<char> v; 
bool check(int pos) {
    if(!tp1) return 1;
    int i, j;
    if(a[pos] > s1[tp1]) return 0;
    if(!tp2) return 1;
    for(i = pos + 1; i <= N; i++) if(a[i] > a[pos] && a[i] > s2[tp2]) break;
    for(int j = i + 1; j <= N; j++) if(a[j] < a[pos]) return 0;
    return 1;
}
int main() {
    N = read();
    for(int i = 1; i <= N; i++) a[i] = read();
    int now = 1;
    for(int i = 1; i <= N + 1; i++) {
        if(a[i] == now) {now++;  v.pb('a'); v.pb('b'); continue;}
        while(now == s1[tp1] || now == s2[tp2]) {
            if(now == s1[tp1]) v.pb('b'), tp1--, now++;
            if(now == s2[tp2]) v.pb('d'), tp2--, now++;
        }
        if(i == N + 1) break;
        if(check(i)) {v.pb('a'); s1[++tp1] = a[i]; continue;}
        if(!tp2 || a[i] < s2[tp2]) {v.pb('c'); s2[++tp2] = a[i]; continue;}
        NO;
    }
    if(tp1 || tp2) NO;
    for(int i = 0; i < v.size(); i++) putchar(v[i]), putchar(' ');
    return 0;
}