1086 Tree Traversals Again (25)

An inorder binary tree traversal can be implemented in a non-recursive way with a stack. For example, suppose that when a 6-node binary tree (with the keys numbered from 1 to 6) is traversed, the stack operations are: push(1); push(2); push(3); pop(); pop(); push(4); pop(); pop(); push(5); push(6); pop(); pop(). Then a unique binary tree (shown in Figure 1) can be generated from this sequence of operations. Your task is to give the postorder traversal sequence of this tree.

\ Figure 1

Input Specification:

Each input file contains one test case. For each case, the first line contains a positive integer N (<=30) which is the total number of nodes in a tree (and hence the nodes are numbered from 1 to N). Then 2N lines follow, each describes a stack operation in the format: "Push X" where X is the index of the node being pushed onto the stack; or "Pop" meaning to pop one node from the stack.

Output Specification:

For each test case, print the postorder traversal sequence of the corresponding tree in one line. A solution is guaranteed to exist. All the numbers must be separated by exactly one space, and there must be no extra space at the end of the line.

Sample Input:

6
Push 1
Push 2
Push 3
Pop
Pop
Push 4
Pop
Pop
Push 5
Push 6
Pop
Pop

Sample Output:

3 4 2 6 5 1

#include<cstdio>
#include<cstring>
#include<stack>
#include<fstream>
#include<algorithm>
using namespace std;

const int maxn=50;
struct node{
	int data;
	node* lchild;
	node* rchild;
};
int pre[maxn], in[maxn], post[maxn];
int n;

node* create(int prel, int prer, int inl, int inr){
	if(prel>prer){
		return NULL;
	}
	node* root=new node;
	root->data=pre[prel];
	int k;
	for(k=inl; k<=inr; k++){
		if(pre[prel]==in[k]){
			break;
		}
	}
	int numleft=k-inl;
	root->lchild=create(prel+1, prel+numleft, inl, k-1);
	root->rchild=create(prel+numleft+1, prer, k+1, inr);
	return root;
}

int num=0;
void postorder(node* root){
	if(root==NULL){
		return;
	}
	postorder(root->lchild);
	postorder(root->rchild);
	printf("%d", root->data);
	num++;
	if(num<n) printf(" ");
}

int main(){
//	freopen("d://in.txt","r",stdin);
	scanf("%d", &n);
	char str[5];
	stack<int> st;
	int x, preindex=0, inindex=0;
	for(int i=0; i<2*n; i++){
		scanf("%s", str);
		if(strcmp(str, "Push")==0){
			scanf("%d", &x);
			pre[preindex++]=x;
			st.push(x);
		} else{
			in[inindex++]=st.top();
			st.pop();
		}
	}
	node* root=create(0, n-1, 0, n-1);
	postorder(root);
	return 0;
}