1020. Tree Traversals (25)

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Suppose that all the keys in a binary tree are distinct positive integers. Given the postorder and inorder traversal sequences, you are supposed to output the level order traversal sequence of the corresponding binary tree.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (<=30), the total number of nodes in the binary tree. The second line gives the postorder sequence and the third line gives the inorder sequence. All the numbers in a line are separated by a space.

Output Specification:

For each test case, print in one line the level order traversal sequence of the corresponding binary tree. All the numbers in a line must be separated by exactly one space, and there must be no extra space at the end of the line.

Sample Input:
7
2 3 1 5 7 6 4
1 2 3 4 5 6 7
Sample Output:
4 1 6 3 5 7 2

IDEA

1.根据后根遍历序列和中根遍历序列，建立二叉树，然后层序遍历二叉树，输出层序遍历序列；

2.用到递归，队列#include<queue>

CODE

#include<iostream>#include<cstdlib>#include<queue>using namespace std;int pos[31],in[31],level[31];struct Bitree{int data;Bitree *left;Bitree *right;};Bitree *buildBitree(int pos_l,int pos_r,int in_l,int in_r){if(pos_l>pos_r){return NULL;}int p=in_l;while(in[p]!=pos[pos_r]){p++;}Bitree *tree=(Bitree *)malloc(sizeof(Bitree));tree->data=pos[pos_r];tree->left=buildBitree(pos_l,p-1-in_l+pos_l,in_l,p-1);//pos_r-in_r+p-1 tree->right=buildBitree(pos_r-in_r+p,pos_r-1,p+1,in_r);return tree;}int levelOrder(Bitree *root){queue<Bitree *>que;Bitree *p=NULL;que.push(root);//将元素插到队尾 int flag=1;int i=0;while(!que.empty()){p=que.front();//访问队首元素 que.pop();// 弹出队列第一个元素，并不会反悔元素值  if(p==NULL){continue;}if(flag){level[i++]=p->data;flag=0;}else{level[i++]=p->data;}que.push(p->left);que.push(p->right);}return i;}int main(){int n;cin>>n;for(int i=0;i<n;i++){cin>>pos[i];}for(int i=0;i<n;i++){cin>>in[i];}Bitree *tree;tree=buildBitree(0,n-1,0,n-1);//由中序，后序建立起这棵树 int len=levelOrder(tree);//层序遍历树 for(int i=0;i<len;i++){if(i==0){cout<<level[i];}else{cout<<" "<<level[i];}}return 0;}

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