Passing the Message 单调栈两次

What a sunny day! Let’s go picnic and have barbecue! Today, all kids in “Sun Flower” kindergarten are prepared to have an excursion. Before kicking off, teacher Liu tells them to stand in a row. Teacher Liu has an important message to announce, but she doesn’t want to tell them directly. She just wants the message to spread among the kids by one telling another. As you know, kids may not retell the message exactly the same as what they was told, so teacher Liu wants to see how many versions of message will come out at last. With the result, she can evaluate the communication skills of those kids.
Because all kids have different height, Teacher Liu set some message passing rules as below:

1.She tells the message to the tallest kid.

2.Every kid who gets the message must retell the message to his “left messenger” and “right messenger”.

3.A kid’s “left messenger” is the kid’s tallest “left follower”.

4.A kid’s “left follower” is another kid who is on his left,
shorter than him, and can be seen by him. Of course, a kid may have more
than one “left follower”.

5.When a kid looks left, he can only see as far as the nearest kid who is taller than him.

The definition of “right messenger” is similar to the definition of
“left messenger” except all words “left” should be replaced by words
“right”.

For example, suppose the height of all kids in the row is 4, 1, 6,
3, 5, 2 (in left to right order). In this situation , teacher Liu tells
the message to the 3rd kid, then the 3rd kid passes the message to the
1st kid who is his “left messenger” and the 5th kid who is his “right
messenger”, and then the 1st kid tells the 2nd kid as well as the 5th
kid tells the 4th kid and the 6th kid.

Your task is just to figure out the message passing route.

InputThe first line contains an integer T indicating the number of test cases, and then T test cases follows.

Each test case consists of two lines. The first line is an integer N
(0< N <= 50000) which represents the number of kids. The second
line lists the height of all kids, in left to right order. It is
guaranteed that every kid’s height is unique and less than 2^31 – 1 .OutputFor each test case, print “Case t:” at first ( t is the case
No. starting from 1 ). Then print N lines. The ith line contains two
integers which indicate the position of the ith (i starts form 1 ) kid’s
“left messenger” and “right messenger”. If a kid has no “left
messenger” or “right messenger”, print ‘0’ instead. (The position of the
leftmost kid is 1, and the position of the rightmost kid is N)Sample Input

2
5
5 2 4 3 1
5
2 1 4 3 5

Sample Output

Case 1:
0 3
0 0
2 4
0 5
0 0
Case 2:
0 2
0 0
1 4
0 0
3 0

题意 是输出每个左边（右边）比他矮的 且最高的人的高度  也就是向左（向右）找到第一个比他高的 然后他到这个人之间的最高的高度 如果没有 输出0 左右各来一遍单调栈 用flag判断是否存在

代码：

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

int stack[50002],l[50002],r[50002],top,a[50002],t,n,flag;

int main()
{
    scanf("%d",&t);
    for(int k=1;k<=t;k++)
    {
        top=0;
        scanf("%d",&n);
        a[0]=a[n+1]=0;
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i]);
        }
        for(int i=1;i<=n;i++)
        {
            flag=0;
            while(top&&a[i]>=a[stack[top-1]])
            {
                flag=1;
                top--;
            }
            if(flag)l[i]=stack[top];
            else l[i]=0;
            stack[top++]=i;
        }
        top=0;///第二次从右往左用单调栈 一定要更新
        for(int i=n;i>=1;i--)
        {
            flag=0;
            while(top&&a[i]>=a[stack[top-1]])
            {
                flag=1;
                top--;
            }
            if(flag)r[i]=stack[top];
            else r[i]=0;
            stack[top++]=i;
        }
        printf("Case %d:\n",k);
        for(int i=1;i<=n;i++)
            printf("%d %d\n",l[i],r[i]);
    }
}