题目传送门：!点我传送!
主要思想：二分图染色
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#include<iostream>
using namespace std;
int Stack_A[1010], Stack_B[1010];
int IN[1010], MIN[1010];
char ANS[10100];
int dui[10100], fa[10100], start[1010];
int col[1010];
void line(int i, int j)
{
dui[++dui[0]] = j; fa[dui[0]] = start[i]; start[i] = dui[0];
dui[++dui[0]] = i; fa[dui[0]] = start[j]; start[j] = dui[0];
}
int boom = 0;
void draw(int x, int c)
{
if (!col[x]) col[x] = c;
else if (col[x] != c) {boom = 1; return;}
else return;
for (int i = start[x]; i != 0; i = fa[i]) draw(dui[i], 3-c);
}
int main(void)
{
int n, i, j;
scanf("%d", &n);
for (i = 1; i <= n; i++) scanf("%d", &IN[i]);
int num = 1; i = 1;
int head_A = 0, head_B = 0;
int sum = 0;
Stack_A[0] = Stack_B[0] = 2000000000;
MIN[n] = IN[n];
for (i = n-1; i >= 1; i--)
MIN[i] = MIN[i+1] < IN[i] ? MIN[i+1] : IN[i];
for (i = 1; i <= n-2; i++)
for (j = i+1; j <= n-1; j++)
if (IN[i]<IN[j]&&MIN[j+1]<IN[i])
line(i,j);
for (i = 1; i <= n; i++)
{
if(!col[i]) draw(i,1);
if (boom)
{
printf("0");
return 0;
}
}
i = 1;
while (num <= n)
{
if (num == Stack_A[head_A])
{
num++; ANS[sum++]='b';
ANS[sum++]=' ';
head_A--; continue;
}
if (num == Stack_B[head_B])
{
num++; ANS[sum++]='d';
ANS[sum++]=' ';
head_B--; continue;
}
if (IN[i] < Stack_A[head_A]&&col[i] == 1)
{
Stack_A[++head_A] = IN[i++];
ANS[sum++]='a'; ANS[sum++]=' ';
}
else
if (IN[i] < Stack_B[head_B]&&col[i] == 2)
{
Stack_B[++head_B] = IN[i++];
ANS[sum++]='c'; ANS[sum++]=' ';
}
else
{
printf("0");
return 0;
}
}
printf("%s", ANS);
return 0;
}