1138 Postorder Traversal (25 分) 给出先序和中序遍历，求后序遍历的第一个结点

超、凢脫俗 2024-04-17 05:47 6阅读 0赞

Suppose that all the keys in a binary tree are distinct positive integers. Given the preorder and inorder traversal sequences, you are supposed to output the first number of the postorder 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 (≤ 50,000), the total number of nodes in the binary tree. The second line gives the preorder 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 first number of the postorder traversal sequence of the corresponding binary tree.

### Sample Input: ###

7
    1 2 3 4 5 6 7
    2 3 1 5 4 7 6

### Sample Output: ###

题意：给出先序和中序遍历，求后序遍历的第一个结点

#include <iostream>
    #include <vector>
    using namespace std;
    int ans=-1;
    int pre[50005],in[50005];
    void posttravel(int inleft, int inright, int preleft)
    {
    	if(ans!=-1||inleft>inright) return;
    	int i=inleft;
    	while(in[i]!=pre[preleft])i++;
    	posttravel(inleft,i-1,preleft+1);
    	posttravel(i+1,inright,preleft+i-inleft+1);
    	if(ans==-1)
    	ans=pre[preleft];
    }
    int main()
    {
    	int n,i;
    	scanf("%d",&n);
    	for(i=0;i<n;i++)
    	scanf("%d",&pre[i]);
    	for(i=0;i<n;i++)
    	scanf("%d",&in[i]);
    	posttravel(0,n-1,0);
    	printf("%d\n",ans);
    }