1138. Postorder Traversal (25)-PAT甲级真题（前序中序转后序）

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:
3

分析：一道简单的前序中序转后序，而且只需要知道后序的第一个值，所以可以定义一个变量flag，当post的第一个值已经输出，则flag为true，递归出口处判断flag，可以提前return
ps：经过测试发现没有flag按照正常前序中序转后序也可以AC，但是我在考场上第一次尝试直接转没有提前退出递归的写法并没有能够AC，而是有一部分运行超时，后来增加了flag才AC。可能pat考试时更为严苛，所以对于准备pat考试的小伙伴，建议如果能够缩小运行时间尽量精益求精

#include <iostream>
#include <vector>
using namespace std;
vector<int> pre, in;
bool flag = false;
void postOrder(int prel, int inl, int inr) {
    if (inl > inr || flag == true) return;
    int i = inl;
    while (in[i] != pre[prel]) i++;
    postOrder(prel+1, inl, i-1);
    postOrder(prel+i-inl+1, i+1, inr);
    if (flag == false) {
        printf("%d", in[i]);
        flag = true;
    }
}
int main() {
    int n;
    scanf("%d", &n);
    pre.resize(n);
    in.resize(n);
    for (int i = 0; i < n; i++) scanf("%d", &pre[i]);
    for (int i = 0; i < n; i++) scanf("%d", &in[i]);
    postOrder(0, 0, n-1);
    return 0;
}

#include <iostream>

#include <vector>

using namespace std;

vector<int> pre, in;

bool flag = false;

void postOrder(int prel, int inl, int inr) {

if (inl > inr || flag == true) return;

int i = inl;

while (in[i] != pre[prel]) i++;

postOrder(prel+1, inl, i-1);

postOrder(prel+i-inl+1, i+1, inr);

if (flag == false) {

printf("%d", in[i]);

flag = true;

}

int main() {

int n;

scanf("%d", &n);

pre.resize(n);

in.resize(n);

for (int i = 0; i < n; i++) scanf("%d", &pre[i]);

for (int i = 0; i < n; i++) scanf("%d", &in[i]);

postOrder(0, 0, n-1);

return 0;

}

❤ 点击这里 -> 订阅《PAT | 蓝桥 | LeetCode学习路径 & 刷题经验》by 柳婼

❤ 点击这里 -> 订阅《从放弃C语言到使用C++刷算法的简明教程》by 柳婼

❤ 点击这里 -> 订阅PAT甲级乙级、蓝桥杯、GPLT天梯赛、LeetCode题解离线版