单调栈模板

0单调栈 - 蓝桥云课

const int N = 2e5 + 10;


int n;
LL a[N];
LL stk[N],top;
LL L[N],R[N];

void solve()
{
	cin >> n;
	for (int i = 1;i <= n;i ++) cin >> a[i];
	
	//第一行输出每个数字其左边第一个比其大的数字，不存在则输出 -1。
	for (int i = 1;i <= n;i ++)
	{
		while(top && a[stk[top]] <= a[i]) top--;
		if (top) L[i] = stk[top];
		else L[i] = -1;
		
		stk[++top] = i;
	}	
	
	for (int i = 1;i <= n;i ++) cout << (L[i] == -1 ? -1: a[L[i]]) << " "; cout << endl;
	top = 0;
	
	//第二行输出每个数字其右边第一个比其大的数字，不存在则输出 -1。
	for (int i = n;i >= 1;i --)
	{
		while(top && a[stk[top]] <= a[i]) top--;
		if (top) R[i] = stk[top];
		else R[i] = -1;
		
		stk[++top] = i;
	}
	
	for (int i = 1;i <= n;i ++) cout << (R[i] == -1 ? -1: a[R[i]]) << " "; cout << endl;
	top = 0;
	
	//第三行输出每个数字其左边第一个比其小的数字，不存在则输出 -1。
	for (int i = 1;i <= n;i ++)
	{
		while(top && a[stk[top]] >= a[i]) top --;
		if (top) L[i] = stk[top];
		else L[i] = -1;
		
		stk[++top] = i;
	}
	
	for (int i = 1;i <= n;i ++) cout << (L[i] == -1 ? -1: a[L[i]]) << " "; cout << endl;
	top = 0;
	
	
	//第四行输出每个数字其右边第一个比其小的数字，不存在则输出 -1。
	for (int i = n;i >= 1;i --)
	{
		while(top && a[stk[top]] >= a[i]) top --;
		if (top) R[i] = stk[top];
		else R[i] = -1;
		
		stk[++top] = i;
	}

	for (int i = 1;i <= n;i ++) cout << (R[i] == -1 ? -1: a[R[i]]) << " "; cout << endl;
}