Rebuilding Binary Tree _ C ++ and binary tree _ c

Rebuilding Binary Tree _ C ++ and binary tree _ c

I. Question Background

Returns the forward and middle traversal of a binary tree.

For more information about binary tree traversal, see

Http://blog.csdn.net/fansongy/article/details/6798278/

Ii. Algorithm Analysis

For example, the binary tree below

Its first traversal is dbacepidermal

Its central order traversal is: ABCDEFG

Its post-order traversal is: ACBFGED

First, point to the first character in the first order, that is, the root node D of the tree.

Then, locate D in the middle-order traversal and divide the traversal into ABC and EFG. Because the Middle-order traversal is based on the structure of the left-right structure, we can see that the left-hand subtree of D is its right subtree, and the right-hand subtree is its right subtree on the left.

Go to its left subtree ABC. At this time, the pointer is + 1, pointing to B

Locate B in the left subtree ABC and divide it into two parts: A and C. A is the left subtree and C is the right subtree.

Pointer correspondingly + 2

This keeps recursion until all nodes are found.

The overall idea is to find the root node of the sub-tree from the first-order traversal, and then recursively traverse the left and right sides in the middle-order traversal. At the same time, each node is added with a pointer + 1 for the first-order traversal, it can be proved that this method is correct

To determine whether a binary tree can be formed, you only need to determine whether the root node can be found. If not, the binary tree cannot be formed.

 1 #include <algorithm> 2 #include <iostream> 3 #include <cstdlib> 4 #include <cstring> 5 #include <cstdio> 6 #include <cmath> 7 #define N 10000 8 using namespace std; 9 10 char mid[N],frt[N];11 int k,cr[N],cl[N];12 int bt(int l,int r)13 {14     if (l>r) return -1;15     if (l==r)16         {17             k++;18             return l;19         }20     int i;21     for (i=l;i<=r;i++)22         {23             if (frt[k]==mid[i])24                 {25                     break;26                 }27         }28     k++;29     cl[i]=bt(l,i-1);30     cr[i]=bt(i+1,r);31     return i;32 }33 void outp(int x)34 {35     if (x==-1) return;36     outp(cl[x]);37     outp(cr[x]);38     cout<<mid[x];39 }40 int main()41 {42     int len,i;43     freopen("bt.in","r",stdin);44     freopen("bt.out","w",stdout);45     gets(frt);46     gets(mid);47     k=0;48     len=strlen(mid);49     for (i=0;i<=len;i++) cl[i]=cr[i]=-1;50     outp(bt(0,len-1));51     cout<<endl;52     return 0;53 }

 

Iii. Question Source

Jiudu Oline Judge question 1385: rebuilding a binary tree (this requires determining whether it can be built)

Http://ac.jobdu.com/problem.php? Pid = 1, 1385

Nanyang Institute of Technology Online evaluation system question 756: rebuilding a binary tree (this is the input of the central order and the post order traversal, find the first order traversal)

Http://acm.nyist.net/JudgeOnline/problem.php? Pid = 1, 756

 

 

 

 

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