题链;http://acm.hdu.edu.cn/showproblem.php?pid=1710

Binary Tree Traversals

A binary tree is a finite set of vertices that is either empty or consists of a root r and two disjoint binary trees called the left and right subtrees. There are three most important ways in which the vertices of a binary tree can be systematically traversed or ordered. They are preorder, inorder and postorder. Let T be a binary tree with root r and subtrees T1,T2.

In a preorder traversal of the vertices of T, we visit the root r followed by visiting the vertices of T1 in preorder, then the vertices of T2 in preorder.

In an inorder traversal of the vertices of T, we visit the vertices of T1 in inorder, then the root r, followed by the vertices of T2 in inorder.

In a postorder traversal of the vertices of T, we visit the vertices of T1 in postorder, then the vertices of T2 in postorder and finally we visit r.

Now you are given the preorder sequence and inorder sequence of a certain binary tree. Try to find out its postorder sequence.

9
1 2 4 7 3 5 8 9 6
4 7 2 1 8 5 9 3 6

7 4 2 8 9 5 6 3 1

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

struct tree
{
	struct tree* l;
	struct tree* r;
	int val; 
	tree()
	{
		l=NULL;
		r=NULL;
	}
}; 

int pre[1010],in[1010];
int n;
tree* root;
void build(int num,int l,int r,tree *rt)//pre num   in_l  in_r
{
	int flag=-1;
	while(flag==-1)
	{
		for(int i=l;i<=r;i++)
		{
			if(pre[num]==in[i])
				flag=i;
		}
		if(flag==-1)
			num++;
	}
	if(flag-l>0)
	{
		rt->l=(tree*)malloc(sizeof(tree));
		*(rt->l)=tree();
		build(num+1,l,flag-1,rt->l);
	}
	if(r-flag>0)
	{
		rt->r=(tree*)malloc(sizeof(tree));
		*(rt->r)=tree();
		build(num+1,flag+1,r,rt->r);
	}
	rt->val=pre[num];
}

void post(tree* nw)
{
	if(nw->l!=NULL)
	post(nw->l);
	if(nw->r!=NULL)
	post(nw->r);

	if(nw==root)
		printf("%d",nw->val);
	else
		printf("%d ",nw->val);

}

void del(tree* nw)
{
	if(nw->l!=NULL)
	del(nw->l);
	if(nw->r!=NULL)
	del(nw->r);
	free(nw);
}

int main() 
{
	while(scanf("%d",&n)!=EOF)
	{
		for(int i=0;i<n;i++)
			scanf("%d",&pre[i]);
		for(int i=0;i<n;i++)
			scanf("%d",&in[i]); 
		root=(tree*)malloc(sizeof(tree));
		*root=tree();
		build(0,0,n-1,root); 
		post(root);
		puts(""); 
		del(root);  
	} 
	return 0;
}