PAT甲级 1138. Postorder Traversal (25)_, you are supposed to output the first number of t-优快云博客

本文介绍了一种通过前序和中序遍历序列来确定对应二叉树后序遍历首个元素的方法。利用递归算法，在输入二叉树节点总数及前序、中序遍历序列的情况下，输出后序遍历的第一个数。

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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 (<=50000), 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:

题目解析：

给出前序和中序，找出后序排列的第一个点。常规题了。

#include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <map>
#include <set>
#include <queue>
#include <vector>
#include <stack>
#include <algorithm>
#include <iostream>
using namespace std;

#define long long ll
const int MAXN = 5e4 + 10;
const int INF = 0x7fffffff;
const int dx[]={0, 1, 0, -1};
const int dy[]={1, 0, -1, 0};

int In_order[MAXN], Pre_order[MAXN];

int Dfs(int Pre_x, int Pre_y, int In_x, int In_y){
    int In_root;
    for(int i=In_x; i<=In_y; i++){
        if(Pre_order[Pre_x]==In_order[i]){
            In_root=i;break;
        }
    }///printf("%d  %d   %d %d\n", In_root, Pre_order[Pre_x], In_x, In_y);
    int left_remain=In_root -In_x;
    int right_remain=In_y-In_root;
    int ans=In_order[In_root];
    if(left_remain){///有左子树
        Dfs(Pre_x+1, Pre_x+left_remain, In_x, In_root-1);
    }else if(right_remain){///无左子树，有右子树
        Dfs(Pre_x+1, Pre_x+right_remain, In_root+1, In_y);
    }else printf("%d\n", In_order[In_root]);
///
}
int main(){
    int n;
    while(~scanf("%d", &n)){
        for(int i=1; i<=n; i++) scanf("%d", &Pre_order[i]);
        for(int i=1; i<=n; i++) scanf("%d", &In_order[i]);
        Dfs(1, n, 1, n);
        ///printf("%d\n", Dfs(1, n, 1, n));
    }
    return 0;
}